In southern Wales’ Vale of Glamorgan, archaeologists flying over a prehistoric settlement they had previously studied got a surprise: the ghostly outline of a Roman villa on the ground inside the older settlement’s boundaries. “We know of Roman villas built within prehistoric settlements elsewhere,” the Royal Commission on the Ancient and Historical Monuments of Wales wrote in a published statement, “but this is a new example.”

Other new discoveries have been popping up across the British Isles over the last few weeks. An atmospheric high pressure system parked over Scandinavia has made the weather hotter and drier than usual across much of northern Europe. The resultant drought creates the perfect conditions for what archaeologists call “crop marks.”

Stone walls or foundations buried beneath a modern landscape tend to absorb more heat than the soil around them. The heat that those buried stones radiate can bake the soil above to a lighter color, so the outlines of ancient buildings and fences show up on the parched surface of modern lawns and fields. The shapes of centuries-old garden beds and paths have emerged from the lawns of several old estates in the UK. At Gawthorpe Hall in Lancashire, the shapes included some details no one had seen before that may be the ruins of even older gardens, dating back to the reign of Elizabeth I.

And the outline of an entire 18th-century mansion, demolished in 1938, has revealed itself in Nottinghamshire, where archaeologists were already excavating the former cellars. The ghostly outline of the house has been visible during previous droughts, but staff working at the site since 1970 say they’ve never seen it in such detail.

Hidden henges and buried barrows

Stone walls aren’t the only buried feature revealed by the drought. In Ireland, for example, Bronze Age people often dug circular ditches around their fortified settlements, with high embankments on the inner side. Over the next 2,500 to 4,000 years, the settlements themselves fell into ruin, and eventually farmers plowed the embankments flat and filled in the ditches with topsoil. That thicker layer of loose topsoil retains more moisture and nutrients than the surrounding soil, so plants growing over the filled-in ancient ditch tend to grow taller and greener than those on either side. In times of drought, like this summer, the contrast is pronounced enough to stand out as crop marks if you’re looking down on the site from above.

That’s exactly what drone operator Noel Meehan of Copter View Ireland was doing earlier this month when he discovered the outlines of nine buried enclosures in County Meath. Based on the shape and arrangement of the outlines, it’s possible that Meehan discovered a cemetery of ancient ring barrows—a circular mound surrounded by a ditch and embankment, built to house cremated remains—near what looks like a settlement. Ring barrows were most common during the Bronze Age and Iron Age in Ireland, about 2,000 BCE to 400 CE, although some are older. Earlier archaeological work in the area had turned up some evidence that a settlement should be nearby, but no one had been able to find it—until now, perhaps.

And just across the River Boyne in Newgrange, drone photographer Ken Williams of Shadows and Stone Photography and Anthony Murphy of tour company and website Mythical Ireland discovered the outline of a previously hidden circular henge in a farmer’s field. In aerial photographs, the outlines of two concentric rings and a clear entrance stand out dark against the lighter background of surrounding crops. It's not far from another well-known henge site, as well.

New digs?

But the only way to be sure of what Meehan, Williams, and Murphy found, or how old the sites are, is for archaeologists to dig in and see what’s beneath the ground. Other excavations unrelated to the drought have taken place along the same stretch of the River Boyne, as archaeologists studied a 5,500-year-old passage tomb. Unfortunately, there’s currently no sign of digs at the new locations happening in the near future.

Meehan reported his find to the National Monuments Service, but cultural heritage agencies in the UK and Ireland have their hands full this summer. In Wales, for example, the Royal Commission’s senior aerial investigator, archaeologist Toby Driver, and his colleagues have been spending long hours in the air to document the emerging outlines of Bronze Age barrows, prehistoric fortified settlements, Roman forts and villas, a medieval cemetery, and other sites. Most of the focus now is on documenting the sites from the air while the crop marks are visible, before they fade back into the fields that now cover them. Further research, and decisions about whether to excavate any of the sites, will come later, as archaeologists take time to analyze the aerial images in more detail.

“The urgent work in the air now will lead to months of research in the office in the winter months to map and record all the sites which have been seen, and reveal their true significance,” said Driver in a published statement.

Most of that urgency comes from the fact that the crop marks are visible only under certain ephemeral conditions. Even as the drought continues, the crop marks will fade with the seasons, like disappearing ink on the landscape. The archaeological sites whose ghostly outlines appeared in southern Welsh fields in early July had begun to fade into invisibility again by July 11, when the early wheat crop was mostly ripened and the contrast of plants growing over ancient ditches became harder to see. At the same time, farther north, the crop marks were just beginning to appear, and the wheat is just now beginning to ripen.

Climate and preservation

“I’ve not seen conditions like this since I took over the archaeological flying at the Royal Commission in 1997,” said Driver. The heat wave is creating a rare set of conditions perfect for aerial surveys, but a preliminary analysis by meteorologists at World Weather Attribution (WWA) says climate change makes events like this more likely.

The researchers compared recorded temperatures at weather stations around northern Europe with historical data from the same stations going back to 1980. Already, the hottest three days of 2018 in northern Scandinavia and western Ireland are about 5ºC warmer than the average for the hottest three days of the year from 1981 to 2010; in the Netherlands, the hottest days of the year so far have been 3ºC warmer than the usual annual maximum.

But how rare is that kind of deviation from the norm? The WWA researchers did statistical analysis to find out, using NASA Goddard Institute for Space Studies data on changes in the global mean surface temperature (GMST). The NASA data provides a good general measure of how much the atmosphere is warming, which let them calculate the odds of a heat wave like this given the current GMST, as well as under past conditions. The three-day peak of the heat wave in Dublin and the Netherlands, for instance, should happen (statistically speaking) once every four to seven years if the GMST holds steady. But under earlier conditions, those temperatures would have been record highs.

“They have simply become more likely due to anthropogenic climate change,” the WWA researchers wrote.

This isn’t the first time that climate change has revealed previously inaccessible archaeological sites, and this summer’s heat wave has given archaeologists in the British Isles a chance to map previously unknown parts of the ancient landscape without any damage to the sites themselves—which means British archaeology is having a pretty good summer.

But a little farther north, melting glaciers in the mountains of Norway are yielding up 6,000-year-old artifacts that begin to degrade as soon as the ice around them melts, so archaeologists risk losing evidence of the past as quickly as they gain access to it. Elsewhere, rising sea levels threaten coastal settlement sites, and there’s a growing sense of urgency among archaeologists in some of the most heavily impacted regions.

A previous version of this story's headline referred to "the UK's hidden past," and it has been corrected to refer to both Britain and Ireland.