This story appears in the February 2016 issue of National Geographic magazine.

In a brightly lit laboratory above the Museum of London Archaeology (MOLA), conservator Luisa Duarte is gently cleaning a large first-century fresco that had been brought into the museum a few days earlier from a construction site on Lime Street, in the heart of the city’s financial district. Workers digging out the foundation for a new 38-story office block had come upon the ruins of an early Roman building. The museum’s experts dated it to around A.D. 60, making this one of the earliest Roman frescoes yet found in London. At nearly ten feet long and more than six feet high, it’s also one of the biggest and most complete.

“Whoever commissioned this was seriously rich,” says Duarte, palette knife in hand, gently prying away clumps of moist earth still clinging to the fresco’s surface. “A wealthy merchant, perhaps, or a banker. Somebody with taste and money and style. This bit of red, for example, appears to be cinnabar, an expensive and rarely used pigment. We come across it occasionally but only on the very finest work.”

Archaeologists believe the fresco adorned a building that was demolished at the turn of the second century A.D. to make way for a grandiose new basilica and forum, the largest the Romans would ever build north of the Alps, larger than St. Paul’s Cathedral is today. Entire neighborhoods were leveled, the rubble used as landfill, and the next generation’s vision built on top. It was the first of many urban renewal projects over the next 1,900 years.

View Images Construction work near Farringdon Station brought medieval London to light. Tests on skeletons of plague victims buried nearby showed that then, as now, London drew people from afar.

Peel back the pavement of a grand old city like London and you can find just about anything, from a first-century Roman fresco to a pair of medieval ice skates—even an elephant’s tooth. As one of Europe’s oldest capitals, London has been continuously lived in and built over by a succession of Romans, Saxons, Normans, Tudors, Georgians, Regency rakes, and Victorians, each of whom added to the pile. As a result the modern city sits atop a rich archaeological layer cake that’s as much as 30 feet high.

The challenge for archaeologists is that London is also a bustling metropolis of more than eight million inhabitants, chock-full of busy streets and skyscrapers and monumental architecture. Opportunities to lift the concrete veil and poke around in the artifact-rich soil tend to be few and brief. But a perfect storm of landmark engineering projects and a building boom in the archaeological heart of London has provided an unprecedented chance to peek beneath the surface and explore the city’s deep past.

The resulting haul of archaeological goodies has been almost overwhelming. They include millions of artifacts covering the vast sweep of human history along the River Thames—from the early Mesolithic, some 11,000 years ago, to the late Victorian, at the end of the 19th century. The discoveries also include the bones of thousands of rank-and-file Londoners who died and were buried in graveyards that were built over and forgotten centuries ago.

Big Dig Archaeologists unearth centuries-old bones beneath London.

“These excavations have provided us with fascinating snapshots into the lives of Londoners through the ages,” says Don Walker, a human osteologist, or bone specialist, for MOLA. “It makes you realize that we all are just small, passing players in a very long-running story.”

One of the earliest chapters of that story came to light after 2010 at the three-acre building site for Bloomberg London, the soon-to-open European headquarters of the Bloomberg financial empire. Located in the ancient ward of Cordwainer, where leather workers had plied their trade since Roman times, a 40-foot-deep excavation pit turned out to be one of the most significant early Roman sites ever found in London.

As the soil was removed, entire street scenes were revealed, complete with timber-framed shops, homes, fences, and yards. Dating from the early 60s A.D. onward, the site was in such an astonishing state of preservation that archaeologists dubbed it the “Pompeii of the north.” More than 14,000 artifacts were found over the course of the excavation, including coins, amulets, pewter plates, ceramic lamps, 250 leather boots and sandals, and more than 900 boxes of pottery.

View Images Massive boring machines like this one—dubbed Victoria in honor of the British queen who oversaw the birth of modern railways—are carving 26 miles of tunnels beneath London as part of Europe’s largest infrastructure project.

“This was the richest haul of small finds ever to come out of a single excavation in the city,” says archaeologist Sadie Watson, who supervised the dig for MOLA. “It’s giving us an unprecedented glimpse into everyday life in Roman London.”

In the trove were nearly 400 rare wooden writing tablets, some of which still displayed legible letters, legal agreements, and financial documents. (Another site yielded shopping lists, party invitations, and a contract for the sale of a slave girl.) The extraordinary preservation is owed to a forgotten little stream called the Walbrook, which flowed through the heart of Roman Londinium on its way to the Thames. Its marshy banks and waterlogged soils preserved almost anything that fell into them.

“Good old English damp,” Watson says, laughing. “Thanks to the Thames and its tributaries, London has one of the best environments for preserving artifacts that anyone could hope to have. Leather, wood, and metal objects that would rot or rust away anyplace else come out of the ground here in amazingly good condition.”

By far the biggest boon to London archaeology has been the $23 billion Crossrail project, the new east-west underground commuter rail link that is both Europe’s largest engineering project and its biggest archaeological dig. Since work began in 2009, Crossrail’s 26 miles of tunnels and more than 40 construction sites have turned up thousands of artifacts and fossils spanning the past 70,000 years.

The largest and most spectacular excavation was launched this past spring in front of the busy Liverpool Street Station. Plans to build an underground ticketing hall meant cutting through the old Bedlam burial ground, the city’s first municipal cemetery. The job entailed exhuming the skeletons of more than 3,300 Londoners; most died in the 16th and 17th centuries, when the city’s streets were often stalked by plague.

Modern Greater London London 1936 London AFRICA To Shenfield (10 mi) London 1886 (Victorian) HACKNEY ISLINGTON CAMDEN Stepney Green Junction Stratford (Site of 2012 Olympics) Liverpool Street Centre Point BT Tower NEWHAM Farringdon TOWER HAMLETS 30 St Mary Axe Paddington KENSINGTON AND CHELSEA To Reading (32 mi) and Heathrow (12 mi) Abbey Wood Big Ben The Shard The O2 Arena SOUTHWARK Londinium A.D. 43-410 (Roman) HAMMERSMITH AND FULHAM Plumstead Portal GREENWICH LAMBETH LEWISHAM WANDSWORTH 2 mi Crossrail stations Crossrail tunnel London Underground Crossrail surface line 2 km The new Crossrail route runs for 73 miles (26 miles underground) through a metropolis that has been growing for centuries. Modern Greater London London 1936 London London 1886 (Victorian) Stepney Green Junction Centre Point 30 St Mary Axe Stratford (Site of 2012 Olympics) BT Tower Abbey Wood Paddington Big Ben The Shard The O2 Arena Londinium A.D. 43-410 (Roman) Plumstead Portal 2 mi Crossrail surface line Crossrail tunnel 2 km Crossrail stations London Underground The new Crossrail route runs for 73 miles (26 miles underground) through a metropolis that has been growing for centuries. FERNANDO G. BAPTISTA AND LAUREN C. TIERNEY, NGM STAFF; VICTORIA SGARRO, DAISY CHUNG. </br>SOURCES: Crossrail; Transport for London; <i>The London Encyclopaedia</i>; Ordnance Survey

With churchyards rapidly filling up with plague victims, city officials decided to establish a public cemetery to accommodate the overflow. The governors of the Bethlem Royal Hospital—popularly known as Bedlam, Europe’s first insane asylum—sold them one acre of land in 1569. Because it was not affiliated with any church, Bedlam became the resting place of choice for radicals, nonconformists, migrants, and misfits, as well as the working poor. By the time the cemetery finally closed, sometime around 1738, it was filled to capacity many times over, with an estimated 30,000 dead buried there.

“The Bedlam burial ground is the most diverse graveyard in the city,” says Jay Carver, Crossrail’s chief archaeologist, whose team spent months researching the site before starting the excavation. “The whole spectrum of society is represented here, from madmen and criminals to the wife of a former Lord Mayor of London.”

Carver and I are standing on a viewing platform overlooking the excavation. In the pit below, a team of 30 archaeologists in orange overalls and blue hard hats are brushing soil from the brows of skulls. Many of the skeletons being exhumed are believed to have perished in the great plague outbreak of 1665, which killed 75,000 to 100,000 Londoners out of a total population of around 450,000.

London Through Time Embark on an animated journey through 40,000 years of London history.

Scientists plan to run tests on some of the remains in hopes of learning about the evolution of the plague bacterium that killed so many. “One of the great mysteries is why the plague never returned to London after 1665,” Carver says. “Up until that time it was a fairly regular visitor to the city, but never afterwards. Why? What changed? We’re hoping this can provide some answers.”

Identifying the remains of individual people in the old Bedlam cemetery is next to impossible. Although some of the coffins had initials on them, tombstones were broken up and reused in walls and buildings when the area was redeveloped. But one set of bones that might be identified is that of Robert Lockyer, a populist radical who was executed by firing squad in 1649. He was buried at Bedlam with the biggest funeral the old graveyard ever saw, attended by more than 4,000 mourners. Carver is keeping a special eye out for him. “If we come across any skeletons with musket-ball holes, we’ll have a pretty good idea whose it is.”

The manner of Lockyer’s passing would give his skeleton a certain historical cachet, but the bones of others may tell a more interesting tale. “Skeletons normally tell us much more about how people lived than how they died,” says Don Walker, the osteologist.

Isotope and bone analysis from a collection of 14th- and 15th-century skeletons unearthed during an excavation at Charterhouse Square paint a harrowing picture of life in medieval London. Many showed signs of malnutrition, and one in six suffered from rickets. Severe dental problems and tooth abscesses were also common, as was a high rate of back injuries and muscle strains from heavy labor. People from the latter period, in the 1400s, had disturbingly high rates of upper body injuries, possibly consistent with violent altercations that resulted from a breakdown in law and order in the wake of the plague.

And yet London still seemed to be a powerful draw for country folk seeking a better life. Isotope analysis reveals that nearly half of the skeletons tested were individuals who had grown up outside the city, some having migrated from as far away as northern Scotland. “It would seem that 14th-century London was already drawing people from all around Britain, just as it does today,” Walker says.

View Images In a single pass a boring machine grinds through tons of rock and dirt, then lines the tunnel walls with concrete, leaving behind a gleaming passage that’s ready for track installation.

It’s eight o’clock on a damp weekday morning, and the sidewalk in front of the Cannon Street Station is bustling with commuters. Few if any notice the iron grille set into the foundation of a former bank building across the street, let alone peek between the bars to see the chunk of limestone that resides there, tucked away for safekeeping. It is the London Stone.

What its original purpose was no one can say, although legend has it that the city will fall if the stone is ever removed or destroyed. It’s mentioned in property deeds dating back to 1108 and was considered an old, old landmark even then. Sixteenth-century antiquarian William Camden believed it was a Roman milliarium, the ground-zero milepost from which all the distances in Roman Britain were measured.

It gets a mention in the plays of William Shakespeare and the poems of William Blake. For centuries it sat in the middle of the street, a folkloric landmark, until 1742, when it was finally deemed a traffic hazard and shifted to the north side of the street, out of the way. There it has remained ever since, at first beside the entrance to St. Swithins Church and later, after the church was destroyed during the blitz, set into a recess in the wall of the new building.

“What the London Stone is supposed to be is a bit of a mystery,” says Jane Sidell, inspector of ancient monuments for Historic England, the national body that champions preservation of landmarks. “But it plays a role in the history of archaeology in London.” When Sir Christopher Wren rebuilt St. Swithins Church, for instance, after the Great Fire in 1666, he made a point of erecting a cupola around the nearby London Stone in order to protect it. This is the first known example of somebody going out of their way to protect an archaeology site in situ.

Wren took rather less care about the substantial Roman ruins he uncovered while digging the foundations for St. Paul’s Cathedral. Fortunately for posterity another man did, a local antiquarian named John Conyers, who followed Wren’s workmen around, taking notes, bagging artifacts, and making detailed drawings in what modern historians regard as one of the world’s first formal archaeological investigations.

Conyers also recorded the excavation of a mammoth a few years later, near Kings Cross, and was the first to argue, successfully, that the flint hand ax found nearby was of human origin. “Previously these sorts of things were said to be ‘faerie thunderbolts,’” Sidell points out.

But it wasn’t until the 1840s, when Victorian engineers began tunneling under the city to build an extensive sewer system, that the newfound science of archaeology found its feet. A pharmacist, coin collector, and amateur antiquarian named Charles Roach Smith cast aside social convention, put on old clothes, and dropped down into the tunnels to follow the workmen. Like Conyers, he observed their digging, took notes, made drawings, and salvaged whatever artifacts he could. “It was the beginning of construction site archaeology as we know it,” says Crossrail’s Jay Carver.

Roach Smith became the nation’s foremost authority on Roman British antiquities, and his book Illustrations of Roman London was the definitive work on the subject for 50 years. His personal collection of artifacts later formed the nucleus of the Museum of London’s own Roman British collection. By a curious quirk of fate, the site of Roach Smith’s former home at 5 Liverpool Street is occupied today by the office block where Crossrail’s archaeology team is based, a coincidence not lost on its chief archaeologist. “Roach Smith occupies a special place in our thinking,” he says. “Although he was working 150 years ago, his observations and notes have been useful in alerting us to the potential of various sites around the city.”

Not all of London’s archaeology is underground. Imposing segments of the original second-century Roman wall that once encircled the city can still be seen in places such as Tower Hill or St. Alphage Garden, or beside the Museum of London itself, where a stretch of Roman wall was exposed by the German Luftwaffe during a night bombing raid in 1940. Park your car in an underground garage nearby and you can nose your bumper up to one of the city’s original gates. Get your hair cut at the barbershop on the corner of Gracechurch Street and Leadenhall Market, and in the basement downstairs you can see an arch support for the second-century Roman basilica.

“But London’s biggest and most visible archaeology site is the Thames, when the tide is out,” says Nathalie Cohen, leader of the Thames Discovery Programme at the Museum of London Archaeology.

It’s just after sunrise on a clear winter’s morning, with the dome of St. Paul’s Cathedral shimmering in the low-angled sunshine. We’re on the Thames embankment just below the cathedral, making our way down a set of algae-covered stone steps to the freshly exposed foreshore. It’s a jumble of water-smoothed cobblestones, roofing tiles, animal bones, crockery, broken bits of clay tobacco pipes, rusty iron, and chunks of thick colored glass that have been rounded and frosted by the relentless action of the tides.

“Almost everything you see here is archaeology,” says Cohen, who points out a Roman-era roofing tile here, a piece of blue-patterned Victorian porcelain there, as we scramble over the uneven ground. “With every tide this gets jumbled up again. It’s never the same twice. You never know what you’ll find.”

Much of the foreshore is accessible to the public and popular with amateur archaeologists and metal detector enthusiasts—hobbyists whose talents and energy Cohen and her colleagues have enlisted to record, monitor, and safeguard protected sites along the foreshore. One of these is Queenhithe, an indentation in the riverbank just below the Millennium Bridge. First recorded in Anglo-Saxon documents of the late ninth century, it was used by ships well into the 20th century.

It’s also the haunting burial site of two Saxon-era women, one of whom appeared to have been executed by a blow to the head from a sword or ax and buried here between A.D. 640 and 780. “This would have been a creepy spot in those times,” Cohen says. “By then the Romans had been gone for more than 200 years, and the ruins of the city would have been overgrown and falling down and very lonely.”

Back at Liverpool Street archaeologists have sifted their way down to the early Roman level of London’s great mound of history. Here, outside the old city walls, in the dark mud that marks a former course of the Walbrook River, they make an intriguing discovery: an old cooking pot with the lid still on it, crammed with cremated human remains. Somebody had buried it along the riverbank nearly 2,000 years ago. Another 40 human skulls, possibly those of executed criminals or rebels, were found nearby.

“We’ve known for a long time that people had found Roman-era skulls along the Walbrook, but we’d always assumed they had been eroded out of a Roman cemetery and washed downstream,” Carver says. But the latest evidence suggests something different. “Looks like we’re going to have to go back over the finds that have been made along here over the past two centuries and rethink what was going on.”

Looking down at the dark line of earth that marked where the vanished river once flowed, with the murmur of London traffic in my ears, I found myself thinking of the opening scene in Heart of Darkness. Joseph Conrad’s narrator, the garrulous seaman Marlow, reminds his listeners as they sit watching the sun set over London, “And this also … has been one of the dark places of the earth.”