The day was departing when we arrived at the old, gray church and its graveyard of tilting headstones. Beyond, the forest hid the ruins of a Norman castle along with—we hoped—one of the Wonders of Britain.

A tall, tidy man with a cap appeared, out with his dog for an evening walk, and Andrew Evans, wearing a dark, swinging overcoat, approached him. “Can I ask you… do you live around here? We’re after the Bone Well… .” The man offered no sign of recognition. “It should be a spring under the castle somewhere.”

“A natural spring?” he replied, unfazed by the suggestion of a watery catacomb filled with skeletons. “There’s a track leading down there on the left. It’s a good bit of a mile.”

“Okay,” said Evans. “That sounds like it.”

The path to the spring went through a gate that shut with a clang and past a field populated with the white, fluffy sheep that speckle the British countryside. “It’s on the map so it can’t be too hard to find,” Evans said, pointing out our position on his phone, where the well was labeled in bright blue. “We’ll turn around the bottom of the castle, and then there it is—‘Boney Well.’”

This particular well was known, in the 19th century, for its trick of regurgitating the bones of fish and frogs, and it was the best lead Evans had in his search for another place, described in our much older guide on this trip, a list of “wonders” compiled a millennium ago. On this list, there is “a well from which the bones of birds are constantly thrown up.” Only, it’s not entirely clear where this wondrous site could be found. Any well with a connection to small animal bones was worth chasing down.

The track led through the gloaming forest, and Evans’s smile took on an eager edge. “I’m excited to see a new one,” he said. “They’re usually just muddy holes in the ground. But maybe it’ll be spectacular.”

He checked his phone again. “Look out for bones.”

The ruins of Richard’s Castle, a preconquest fortification built by a Norman knight. James Lancaster/Castles Forts Battles UK

In the second century B.C., the Greek writer Antipater of Sidon catalogued the wonders of the ancient world—the Hanging Gardens of Babylon, the Colossus of Rhodes, the Great Pyramid at Giza—and ever since writers have collected accounts of amazing places. When, sometime between the 9th and 12th centuries, a native son took up the task of listing the Wonders of Britain, he included a lake with 60 islands in it and a fountain of salt, a levitating altar and a shape-shifting royal burial mound—all told, 26 natural phenomena and small miracles.

These wonders were concentrated into two areas of Britain—in the north, toward Scotland, and to the west, in what’s now Wales—places where Celtic tribes still held sway after years of Saxon incursions had eroded their territory. The list’s unknown author came from those lands, probably the Welsh border region, and though he was writing in a time of rising Saxon power, his heart seemed to lie with Celtic traditions that were in danger of disappearing.

This is “Dark Age” history, often overlooked in the rush from the Romans to the Renaissance, with details forgotten or recorded only in legend. The Wonders of Britain, too, have disappeared from memory. According to the manuscript curators at the British Library, “few actual geographic features” known today match the list’s descriptions.

Even in the 21st century, a medieval travel guide might still lead to places with the capacity to amaze.

But if the broad outlines of medieval political divisions linger over modern Britain, some of the wonders are still hiding there, too. Evans, a senior lecturer in the geography department at the University of Leeds and self-proclaimed “expert in nothing,” started trying to track them down more than a decade ago. “We get into the rut of drifting through places without really thinking about them,” he said. But finding the site of a medieval wonder can burnish a familiar landscape with a sheen of the strange and mysterious.

Some of the wonders have proven impossible to find; perhaps they never existed in the first place. Others have lost their luster, but, Evans assured me when we first spoke, “Some of them are active and still really spectacular.” The Severn Bore, for example—a rushing, riverine tidal wave that can reach more than nine feet in height—plays a role in four of the wonders and can still be seen.

So it was that a few months later, we were roaming the countryside of England and Wales, hopping fences, tromping through heather, and wandering graveyards, in the hope that, a thousand years on, not all the wonder of the Dark Ages had disappeared. Even in the 21st century, a medieval travel guide, we imagined, might still lead to places with the capacity to amaze.

Andrew Evans at Carngafallt. Sarah Laskow

The trees opened into a clearing, and our path crossed the trickle of water coming from the Boney Well. It was not the hole-in-the-ground variety of well, but a welling up of spring water, little more than a damp channel through the trees. Nothing about the place seemed to justify putting it on a map, modern or medieval, wondrous or mundane.

But, there, among mossy roots, I spotted a spread of gray feathers and a few delicate bones.

“Oh my god,” Evans said, laughing. “That’s conveniently dead in the right place.” He had not expected to find fresh bones at the Boney Well, regardless of how they got there. The list’s description conjured an image of a macabre and magical place, but Evans was searching for a natural wonder, not something from a fairy tale. As fantastic as some of the wonders on the list seemed, they usually had some sort of grounding in reality, or at least in the possible.

The wonders list comes from a medieval text, Historia Brittonum, that was once treated as a reliable account of the history of Britain. It begins with the descendants of Trojan refugees settling on the island and includes one of the earliest known references to King Arthur, along with catalogs of battles between Britons and Saxons and genealogies of forgotten rulers. For centuries, historians and classicists relied on the Historia’s account of the years between the end of the 300s, when the Romans began to lose control of Britain, and the mid-800s, the earliest the Historia could have been written. Now, though, scholars consider these stories more legend than fact.

Like the rest of the Historia, the wonders list, which is tacked on the end of some versions of the manuscript, has parts that can be verified. It includes, for instance, the famous Roman-built baths in Somerset, which draw from the island’s only geothermal spring. But other wonders—the levitating altar, supposedly held up by the will of God, or the returning wooden plank, which floats down the River Severn before reappearing, three days later, in its original place—sound like they belong to the realm of fantasy.

The Roman baths at Somerset. Julian P Guffogg/CC BY-SA 2.0

The presence of the surviving wonders creates the tantalizing possibility that even the fantastical, forgotten ones could have scientific explanations and may have left behind clues that could corroborate their existence. “Given that some of the wonders exist, chances are that all of them once existed,” says Evans. At the start of his search, he decided to seek out plausible explanations for each wonder, even the stranger ones. “Your first impression is that someone has drawn together a whole bunch of crazy things, and part of me thought—why would you want to dig down and understand this?” he says. But the list’s unknown author does seem to be trying to explain the unusual phenomena he’d encountered in his travels. “This person is genuinely curious about the world. And, occasionally, the interactions in the natural world do throw up completely weird stuff.”

The Boney Well, for instance, was once more impressive than it first appeared to us. It seemed that the well had been capped: The lay of the land indicated the stream could have once flowed strong and deep here.

As we were leaving, Evans showed me some of the rocks that he had been turning over in the stream. Embedded in them were shell fossils—bones, of a sort. He had a hypothesis. Perhaps the bone well had been churning through a fossil bed and bringing pieces to the surface. If fossils weren’t common in the area and a singular spring kept piling them up, he said, “Then you’d think, ‘How exciting.’”

In search of the Boney Well. Sarah Laskow

Evans first encountered the Wonders of Britain while searching old Welsh texts for a name to give his eldest child. His wife comes from a Welsh family, and he’d fallen for Welsh literature at Aberystwyth University while studying geology and geography. His interest in folklore is a sideline: He did graduate work as a glaciologist in Iceland and Antarctica and now focuses on human geography, drawing on computer and social sciences to understand how people relate to each other and their environments.

It’s a good field for people with roving minds, like Evans. He’s had at least a passing interest in cathedrals and castles, literary theory and jazz, film history and British culture, voodoo and still-life drawing. He belonged to a caving club in college and after graduation traveled Europe with friends, seeking out ossuaries and cave paintings. Now he lives on the edge of a moor in Yorkshire and keeps a hippo skull in his office. One morning at breakfast he was reading about the first witchcraft trial in Ireland and mulling how concepts we use to understand the world, such as sin and crime, are not always as timeless as they might seem; someone had to think them up. But that’s why the wonders are worth seeking out—using the medieval list as a guide to natural phenomena means “taking whatever tired, dust-covered metaphors you use for the world and shaking them up,” he says.

We set out that morning across the Welsh countryside with soft sunlight peeking from behind the clouds. This part of the world served as a model for J.R.R. Tolkien’s idyllic Shire; it is William Blake’s “green & pleasant Land” of rolling hills and pastures, which transformed, mid-Wales, into craggier peaks. We drove through a forest where moss padded the road’s center like a carpet and then followed a footpath through the woodland pastures of a nature reserve, under the crooked branches of Cornish Oaks and up a steep slope of prickly green gorse. On the crest of the hill, we reached a trio of Bronze Age cairns, tombs that had once protected dead men and their treasures, rising from the brush low and wide like islands from the sea.

Atop one large cairn, made of rough and irregular rock, Evans began to read aloud. “‘There is another wonderful thing in the region which is called Bucit. There is a mound of stones’—So it mentions the cairn—‘and one stone placed on top has a footprint of a dog on it. … When hunting the porker Troynt, Cabal, who was the dog of the soldier Arthur, stamped his step in the stone, and afterward Arthur gathered together stones … and it is called Carn Cabal.’”

The cairns have overlooked these empty hills for millennia. Sarah Laskow

“So,” he said, “we need to find a footprint.” Ideally, an oversized, mythic footprint belonging to King Arthur’s favorite dog, Cabal.

Early Arthurian stories have an otherworldly quality, even stranger than the playful magic of Merlin, which was added to the legend later. Sir Kay fights giant sea-cats and werewolves; he can go nine days and nights underwater without sleeping or breathing. Another companion, Bedwyr, is a handsome, one-handed knight with a magical lance. Arthur has men who can suck up a sea, emit sparks from their feet, level a mountain, speak every language, and hear an ant move from 50 miles away. One, when sad, lets his bottom lip droop below his waist and turns his top lip over his head. Gwenhwyfar, Arthur’s wife, is the daughter of a giant, and gallant, skilled, adulterous Lancelot is nowhere to be found.

During the 12th and 13th centuries these Celtic tales traveled to France, where the older stories disappeared from Arthur’s legend and Lancelot joined the court, now sopped in chivalric romance. Centuries later, 19th-century British antiquarians rediscovered the old Arthur in medieval Welsh manuscripts that, like the Historia, drew from older oral traditions. Carn Cabal is linked to the tale of the hunt for a boar named Troynt (or Twrch Trwyth, Trwyd, Troit, or Terit, depending on whom you ask), who is actually a cursed prince transformed into a wild animal. Culhwch, a cousin of Arthur, falls in love with the daughter of a giant, and to marry her he must complete 40 impossible tasks, including grabbing a pair of scissors, a comb, and a razor stuck to Troynt’s head.

The conglomerate rock of the cairn features paw-like shapes. Sarah Laskow

According to the wonders list, Cabal stepped on a stone and left a mark during that chase. That rock was last reported seen in the 1830s or ’40s, when Lady Charlotte Guest, a linguist and literary scholar, sent an unnamed gentleman up here to Carngafallt—the Rock of Cafall, a variant of Cabal—in search of it. The mountain matches the place described in the wonder.

Lady Guest’s gentleman found what he thought could be “the identical object referred to” in the wonders list—a hefty rock with an impressive mark, four inches long and two deep, sunk into it. “Some unimaginative geologist may persist in maintaining that this footprint is nothing more than the cavity left by the removal of a rounded pebble,” he wrote. There is truth in that view: When fragments of this conglomerate rock work loose, they leave behind gaps that can resemble a paw print. But according to the gentlemen, “Such an opinion scarcely requires a remark.”

Evans, though trained in geology, is not without imagination. We started looking for the shape of a dog’s footprint and in just a few minutes found a half-dozen candidates. With the “eye of faith,” as Evans puts it, these naturally occurring shapes looked distinctly canine in origin, though none were as large as what Guest’s gentleman described.

A path though Cornish oaks. Sarah Laskow

That gentleman wrote that a man could “without any great exertion” carry the rock away, and it seems he might have done just that. Now this silent place, populated mostly by migrating birds, is protected from development. But in centuries past the cairns were broken open; any treasure left inside is long gone. A 19th-century gentleman might not have hesitated to remove a wondrous rock.

Or perhaps Guest’s man wanted to test it. The wonders list also stated that, if the stone were moved from its spot, it would reappear atop the cairn the next day. Even if that magic failed, in this lonesome place, no one would have noticed the rock was missing.

“When I started engaging with early Welsh poetry again, one of the things that struck me was how much of British history has been wiped off the school curriculum,” Evans says. On his website, he writes that the Wonders of Britain “act as pins, fastening Britain today to a hidden landscape of dark age mythology.” The giant dog and cursed boar-prince may have been myths, but the cairns and their pockmarked rocks are not. The people who lived here a thousand years ago thought this place was special, as had Bronze Age people thousands of years before that. Without the text to hint at these layers of history, today it would be a pleasant nature reserve, lovely enough but lacking the draw of a graveyard linked with Britain’s legendary king.

By the time Historia Brittonum was written, the Celtic world it described was already fading into obscurity. Y Gdoddin, a medieval poem dated to roughly the same period, describes one of the Britons’ last-ditch efforts, in the seventh century, to oust the Saxons from their lands. Whatever its historical value, “it’s actually about the most British thing that’s ever been written,” says Evans. “They go to Edinburgh and get drunk for a whole year, saying—eventually, we’re going to go kick the Anglo-Saxons out. Then they lose. They lose terribly. The British have this real delight in heroes who fail, and this is the first example of it.”

By the ninth century, Anglo-Saxon influence was creeping over the whole island. Historia Brittonum itself is evidence of that. Some of the text’s main sources are English, not Welsh, and it’s the information that comes from those Anglo-Saxon accounts that’s now considered most reliable. Over time, any corroboration of the history passed down from Welsh sources was lost. Barring a series of surprise discoveries of lost documents, the full facts of those medieval centuries will never be known for sure.

Landscapes can stand up to time better than human records, but some of the wonders have clearly been lost for good. A pond with different species of fish inhabiting each corner is probably filled in. A northern island of swimming birds could refer to a seabird colony, but there’s little clue as to which island it might have been. Another wonder, the miraculous tomb of Arthur’s son, was likely once in the town of Wormelow Tump, but in the 20th century, perhaps even earlier, locals leveled the substantial burial mound. Now the only features there are a gas station, a pub, and a bus stop.

Wormelow Tump is now a featureless space. Sarah Laskow

On a rainy morning, Evans and I went to see the forgotten site of Linn Liuan, a whirlpool that sucked in the sea then burped it back up. The wonders list indicates that it was somewhere along the Severn, the longest river in Britain. After searching up and down its banks, Evans had lost hope of finding the whirlpool, until one day in 2006, when he received an email from a stranger, John Nettleship, who had read Evans’s online description of the wonder and had an idea where it could have been.

Nettleship, who died in 2011, had been a strict and short-tempered chemistry teacher, and it’s said that one of his pupils, J.K. Rowling, modeled Severus Snape—Harry Potter’s potions teacher—after him. “He was very dry,” Evans says. “And quietly socially conscious.” Nettleship belonged to a small historical society, and he described strange geographic features known as the Whirlyholes, now-dry pools that, according to local lore, used to empty suddenly of water, leaving behind dangerous sinkholes.

Records didn’t place the pools on the river’s current course, but it was the best lead Evans had found. Nettleship had started interviewing local farmers, and together they searched local libraries and scrutinized old maps, looking for evidence that the Whirlyholes were once on the river. The nearby village of Caerwent used to be on the river’s edge; there’s even an old Roman wall there called the “port wall.” With the help of a student, Evans and Nettleship were able to show that the water once reached the Whirlyholes’ location. Locals remembered being warned about the dangers of these pools, which had lingered in a diminished state until the 1970s. Their description of the holes also matched Evans’ hunch about the whirlpool, that it was connected to underground caves that store and release water, making the outflow unpredictable. By 2008, when they published their findings, Evans and Nettleship were convinced they had found the wonder’s most likely location.

The Whirlyholes depression to the right of an overpass. Sarah Laskow

Evans had warned me that there was not going to be much to see. The river moved over time, and a Victorian-era infrastructure project had altered the hydrogeology of the area for good. But he led the way down a footpath and through a charming arch to a highway underpass, where horses had gathered to get out of the rain. Pointing out over the field, he indicated a slight indent in the ground, a dip in the landscape that one would be hard-pressed to notice.

“Now, how would you know it was there?” Evans said. “It’s a shallow bit of farmland.” A once-spectacular whirlpool had become a field where sheep could graze safely.

Perhaps the only wonder from the list that maintains a truly exalted status is the famous Roman spa at Bath, which is guarded today by paved streets and international chain stores instead of mud and sheep. Unlike the others, this wonder has an audio guide, which explains that the baths were “hidden for most of the 2,000 years they’ve been here.”

In the baths, ghostly figures of Roman reenactors are projected onto the rooms’ walls. There’s no need to imagine the past here, as one ghost figure rests on a bench or another receives a massage. The natural wonder of the geothermal waters that feed the baths is mentioned only in passing. And the Wonders of Britain make no appearance. It’s as though the medieval period never happened. In the audio guide version of the story, there are only two important moments in its history: when Romans built the baths and when 19th-century archaeologists rediscovered them.

Standing on the 2,000-year-old stones of the Roman baths, and dipping a finger into the warm mineral water, did transmit a touch of awe. But the experience of standing in Caerwent, looking out from the port wall to see the one-time path of the River Severn and finding the dips of the dried-up Whirlyholes, while not spectacular, had its own power. The wonders list can act as a decoder for the landscape, revealing secrets in a nondescript underpass, a featureless field, an ordinary intersection. These places might seem like they have no history, but once they were remarkable.

Hills of springy heather and gorse lead to the cairns. Sarah Laskow

Not everyone has the same enthusiasm for tromping around faded footpaths in the English and Welsh countryside that Evans does. He knows that. But he was right when he said that it would all be worth it. On the first night that I met him, before we went to the sites of Carn Cabal, Linn Liuan, or the Bone Well, we went to see the Severn Bore.

A bore is a tidal phenomenon—a flood tide gathers into a wave that rushes up a bay or river, against the current. The shape of the river bed channels the incoming tide along a path that narrows so quickly the water rises into a wave that speeds up the river, as fast as 13 miles per hour on the Severn. These waves can be dramatic, and surfers try to ride them upstream. (In 2006, one rode the bore for 7.6 miles, a world record at the time.) The bore comes only a handful of times each month, as it has for centuries, though rarely with a strength that makes it worth seeing. The most powerful bores usually come around the spring and autumn equinoxes; Evans and I visited in early November, for a bore that was predicted to rate three stars out of five. But the innkeeper at our hotel, who had wished us well on our “boring” adventure, said that the waves are unpredictable. A predicted four-star bore might pass with a whimper, a three-star might roar by like a pack of motorcycles on a mountain highway.

The Severn Bore. Galaxy Picture Library/ Alamy

The River Severn winds through the landscape in long S-curves, so it’s possible to see the bore multiple times as it travels upriver. The first place we waited to see it, near Bristol, the river spread wide, and dark mud banks sat low in the water. In the distance, the lights of the city glowed, and the fireworks of Guy Fawkes Night popped now and again across the still water.

After a long while, I heard a roar in the distance, like a muffled airplane engine, growing louder.

“It’s pretty scary, isn’t it?” said Evans. “Like a tsunami.”

We heard the sound of water lapping at the shore grow agitated. Then—an unmistakable, fair-sized wave, cruising across the water.

We raced upriver and within minutes were standing on a bend where a pub was blasting Journey’s “Don’t Stop Believin’.” The bore approached again, this time as a line drawn across the river, impossibly straight. The wave was taller and faster: Before it, the river was flat, black, and glassy and then, a moment later, three or four feet higher, rough and aggressive. It was like looking at two different landscapes at once, a set of before-and-after photos spliced together.

“Want to see if we can catch it farther back?” Evans said.

We were off again, like Arthur’s men chasing the enchanted boar, giddy and intent. We pulled off a road lined with hedges, the river just on the other side, narrower here. Where the road bent away from the river, we found a footpath leading to a break in the trees. Again came a low rumble roiling to a crescendo.

It felt dangerous, in the dark night, by the muddy, cold river, too close to a wave powerful enough to knock a person down and snatch them away.

“It’s getting scarier as it gets louder, the waves right on the edge, chopping up against that …” Evans was saying, when the bore rounded the corner, crashed against the curve of land where we had stood, and zoomed upriver. The water rushed up the bank, and we both scrambled back from edge before the surge could pull us in. It felt dangerous, in the dark night, by the muddy, cold river, too close to a wave powerful enough to knock a person down and snatch them away.

This bore would not have come as a surprise to medieval people. Many of the Wonders of Britain were trees, rocks, and springs because those natural features dominated the landscape. Traveling meant spending long stretches on roads that led far from human places, or depending on the rhythms of the river. They would have tracked the tides and known when the bore might visit, even if it was weak when they thought it would strong, roaring when they expected it to be subdued. Like anything wild, it can be studied and better known, but remains in some ways unpredictable. A thousand years after the Wonders of Britain were recorded, they still have the power to surprise. I have never seen anything like it.