For thousands of years, fields across Britain have been hiding the remains of settlements, burial monuments, ancient boundaries and ceremonial landscapes.
Most of the time, they are almost impossible to see.
Then the weather changes.
During prolonged periods of hot, dry weather, crops growing above buried archaeology can behave differently from plants growing over undisturbed ground. What appears to be an ordinary field from ground level can suddenly develop strange lines, circles, rectangles and geometric patterns when viewed from the air.
In 2018, an exceptionally dry summer produced some of the clearest archaeological cropmarks seen in years.
Historic England's aerial archaeologists took to the skies — and discovered that parts of England's prehistoric landscape were far richer than previously realised.
When the landscape gives itself away
The principle behind these discoveries is surprisingly simple. Archaeological remains buried beneath a field can alter the soil above them.
Ancient ditches may have filled with richer, deeper soil. Crops growing over them can therefore have access to more moisture and nutrients than plants elsewhere in the field.
Buried walls, foundations and compacted surfaces can have the opposite effect. Their presence may restrict roots and reduce the amount of moisture available to crops.
During normal weather, these differences may be almost invisible.
During a drought, however, the contrast can become dramatic. The result is a kind of natural archaeological photograph.
From an aircraft, archaeologists may suddenly see outlines of structures that have not been visible on the ground for centuries — sometimes even millennia.
Historic England describes these features as cropmarks, soilmarks and parchmarks, and aerial reconnaissance has become one of the principal methods used to identify buried archaeological remains across England.
The summer when the past appeared
The summer of 2018 provided exceptional conditions. After prolonged hot and dry weather, crops across parts of England began revealing buried archaeology with unusual clarity.
Historic England's aerial archaeologists reported discoveries including prehistoric settlements, burial monuments, Iron Age sites and Roman farms.
Among the most intriguing finds were Neolithic cursus monuments — enormous linear monuments dating broadly to between about 3600 and 3000 BC.
The discoveries demonstrated something important.
Archaeological landscapes are not necessarily lost simply because their monuments have disappeared from view.
The evidence may still be underground. It may merely be waiting for the right combination of soil, crops, sunlight and weather to reveal itself.
The mysterious cursus monuments
Near Clifton Reynes in Milton Keynes, aerial survey identified two Neolithic cursus monuments.
These are among Britain's most enigmatic prehistoric structures.
A cursus generally consists of two roughly parallel ditches enclosing a long, narrow strip of land. Some are comparatively short, while others extend for extraordinary distances.
The famous Dorset Cursus, for example, stretches for nearly 10 kilometres.
Nobody knows exactly what cursus monuments were built for.
They have been interpreted as possible processional routes, ceremonial spaces, territorial boundaries or monuments separating different parts of the landscape.
The structures near Clifton Reynes are particularly interesting because one had previously been obscured by a medieval field boundary or headland.
Only when the conditions became exceptionally dry did the buried archaeology become clearly visible from above.
It is a reminder that even landscapes that have been farmed for generations can still contain prehistoric monuments waiting to be rediscovered.
Cornwall's hidden settlements
The drought also produced discoveries in Cornwall. At St Ive, aerial photography revealed the remains of an Iron Age settlement known as a round.
Cornish rounds were enclosed settlements generally consisting of a circular bank and external ditch, with an entrance and roundhouses positioned within the enclosure.
The example revealed from the air was even more interesting because other archaeological features appeared nearby.
A rectangular enclosure, a double-ditched enclosure and a small circular feature — potentially the remains of a Bronze Age barrow — were also visible.
The combination suggests that this was not simply a single isolated settlement. Instead, it may represent a landscape used repeatedly over thousands of years.
Lansallos: an unusual prehistoric enclosure
Another Cornish discovery appeared near Lansallos. From the ground, there was little indication of what lay beneath the field.
From above, however, an unusual arrangement emerged.
The site contained concentric ditches, with entrances connected by two parallel ditches.
Historic England suggested that the inner enclosure may have surrounded a Bronze Age or Iron Age settlement, while the outer ditch could have been constructed later and used as part of a system for controlling livestock.
The interpretation remains provisional.
That uncertainty is typical of aerial archaeology.
A cropmark can reveal the shape of something that has disappeared, but it cannot automatically tell archaeologists precisely when it was built or how it was used.
Those questions often require geophysical survey, excavation and dating.
The square barrows of Yorkshire
The Yorkshire Wolds produced another striking discovery. Four square cropmarks near Pocklington indicated the buried remains of Iron Age square barrows.
Unlike the familiar circular burial mounds scattered across Britain, these monuments were surrounded by square or rectangular ditches.
The Yorkshire Wolds contain an unusually high concentration of such monuments, some associated with elaborate burials and exotic grave goods, including chariots.
Their discovery during an exceptionally dry period demonstrated that even archaeological landscapes considered well studied can still produce surprises when environmental conditions are favourable.
And this is perhaps one of the most fascinating aspects of drought archaeology. The discoveries aren't necessarily occurring in places nobody has ever investigated.
Sometimes they are appearing in landscapes that archaeologists have examined for decades. The difference is simply that the ground has finally decided to reveal something new.
A strange triple-ditched monument in Derbyshire
Near Scropton in Derbyshire, aerial photographs revealed an unusual circular feature surrounded by three ditches.
Archaeologists interpreted it as a complicated Bronze Age burial mound or barrow. Nearby, another pattern appeared: a line of pits crossing the field.
Such pit alignments are characteristic of parts of the Midlands and are generally interpreted as prehistoric boundaries, although their precise dates and functions can vary.
The same photograph also captured medieval ridge-and-furrow agriculture and natural geological features.
In a single aerial image, therefore, archaeologists could see traces of several different periods of human activity — and the natural landscape that had influenced them.
The buried farms of Somerset
The dry conditions also exposed evidence of prehistoric farming near Stogumber in Somerset. Several sites were identified as probable Bronze Age and Iron Age farms.
Some appear to represent different phases of settlement, with one enclosure apparently abandoned before another was constructed over or alongside it.
This creates an extraordinary possibility.
A modern field may occupy exactly the same piece of landscape where prehistoric communities farmed, kept animals and built their homes thousands of years ago.
The buildings themselves have disappeared. The people who lived there are forgotten. But the ditches they dug remain beneath the soil, silently preserving the shape of their world.
A Roman farm appears
Not everything revealed by the drought was prehistoric. Near Bicton in Devon, cropmarks exposed what appears to be a Roman-period farm.
Different phases of activity could be distinguished from the aerial imagery. A central enclosure may have contained buildings, while additional enclosures and paddocks were subsequently attached to it.
This demonstrates another advantage of aerial archaeology.
Rather than revealing a single building, aerial photography can expose an entire landscape. Fields, tracks, settlements, boundaries and agricultural areas can all become visible at once.
The result is often less like discovering a building and more like finding a forgotten map.
Eynsham's prehistoric landscape
Near Eynsham in Oxfordshire, aerial photographs revealed a complex prehistoric ceremonial landscape. The site was already known and protected as a scheduled monument, but the exceptional weather brought additional features into view.
Among them was a previously elusive circle of pits. Other cropmarks revealed funerary monuments and settlement activity dating broadly from the later prehistoric period.
The discovery illustrates an important point: drought does not only reveal completely unknown sites.
It can also expose previously hidden parts of archaeological sites that researchers already know exist.
A familiar monument can therefore become a new archaeological puzzle.
More than 50 discoveries in northern England
The 2018 drought was not confined to southern England. Historic England's aerial team flew extensively across the North, beginning in Lincolnshire and the Yorkshire Wolds.
More than 50 archaeological sites were recorded for the first time on the chalk landscapes surveyed that summer.
These included Neolithic long barrows, Bronze Age round barrows, Iron Age square barrows and settlements from the Iron Age and Roman periods.
Some of the areas were already regarded as archaeologically important. Yet the aerial survey demonstrated that even heavily studied landscapes can still contain previously unrecorded remains.
Why don't we see these places every year?
One of the strangest things about cropmark archaeology is that a site can appear spectacularly clear one summer and virtually disappear the next.
Weather conditions matter enormously. Soil moisture, crop type, geology, agricultural practices and the precise timing of rainfall can all influence whether buried archaeology becomes visible.
Historic England notes that cropmarks may appear or disappear from year to year depending on these conditions.
This means archaeology from the air is partly a race against time. When conditions are right, survey aircraft need to be in the air.
A few weeks later, the evidence may have vanished again.
The technology behind the discoveries
Aerial archaeologists don't rely exclusively on aircraft. Modern archaeological investigation combines numerous forms of remote sensing.
Historic England uses aerial photography alongside LiDAR, geophysical survey, mapping and ground investigation.
LiDAR is particularly useful because laser scanning can reveal subtle variations in ground level, even where vegetation or modern land use makes them difficult to see.
Aerial photographs, meanwhile, can reveal buried remains through changes in vegetation.
By comparing imagery captured in different years and under different conditions, archaeologists can build a much more complete picture of a landscape.
A landscape full of ghosts
There is something extraordinary about looking down at an apparently ordinary English field and realising that its geometry may have nothing to do with modern farming.
A faint rectangle might represent a prehistoric enclosure. A circle could be a vanished burial mound. Parallel lines might mark an ancient boundary. A series of small pits could preserve the remains of a prehistoric land division.
And an apparently empty field may conceal the footprint of an entire settlement.
The landscape we see today is therefore only the latest layer. Beneath it are thousands of years of activity.
The drought paradox
There is an uncomfortable side to these discoveries. The same environmental conditions that reveal archaeological sites can also threaten them.
Historic England warns that archaeological remains can be vulnerable to agricultural activity, development and other forms of landscape change.
In some areas, buried archaeology has already been damaged or levelled by centuries of cultivation.
Drought can expose the evidence, but it does not necessarily preserve it. And as climate patterns change, archaeologists face a complicated future.
More frequent or severe summer droughts could create additional opportunities to identify buried sites — but could also put some archaeological deposits at greater risk.
In wetland environments, for example, falling water tables can threaten waterlogged prehistoric archaeology that depends upon saturated conditions for preservation.
The weather can therefore act both as a detective and a destroyer.
Britain's invisible archaeological landscape
Perhaps the greatest lesson from the drought discoveries is that Britain's archaeological landscape is far from completely mapped.
Historic England's aerial programmes have added thousands of previously unknown sites to the archaeological record. Between 2011 and 2014 alone, more than 14,000 previously unknown sites were added through aerial survey projects conducted to National Mapping Programme standards.
The numbers are remarkable.
They suggest that the archaeological map of Britain is still being rewritten — not necessarily through spectacular excavations, but through fleeting glimpses from the sky.
A dry summer can turn an ordinary field into a historical document. For a brief moment, buried ditches become lines. Lost settlements become circles. Ancient monuments emerge from the crops.
Then the rain returns.
The field turns green again. And the landscape closes over its secrets.
But somewhere beneath the soil, the remains of prehistoric Britain are still there — waiting for another summer when the past decides to show itself.
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