The current heatwave is providing a near-unprecedented bonanza for archaeologists, as scorched conditions all over Britain expose the previously undiscovered or long-hidden outlines of everything from ancient fortifications to remnants of the Second World War.

In what was described as “a frantic race against time and weather”, archaeologists are scrambling into aeroplanes or flying drones to search for the outlines which are visible from the air as “crop marks”, before they are once more erased by rain.

In Wales alone the new discoveries have included an early medieval cemetery in south Gwynedd, a Roman villa in the Vale of Glamorgan, a prehistoric or Roman farm near Newport and a Roman fortlet near Magor, south Wales.

Members of the public are spotting the signs of everything from Bronze Age burial grounds in their local park to long-forgotten Second World War air raid shelters in back gardens and schools.

And for the professionals, something akin to archaeological gold-rush fever has set in.

“It’s hugely exciting,” said Louise Barker, a senior archaeological investigator at the Royal Commission on the Ancient and Historical Monuments of Wales (RCAHMW). “There have been whole new discoveries, covering all periods of time.

“Our senior aerial investigator Dr Toby Driver is flying all over Wales, going over landscapes and saying, ‘Oh my goodness, there is something I never expected down there.’ He says so much new archaeology is showing it is incredible.

“There probably hasn’t been anything like this for more than 40 years. It is pretty spectacular.”

Historic sites across UK exposed by heatwave Show all 15 1 /15 Historic sites across UK exposed by heatwave Historic sites across UK exposed by heatwave Crop marks show of the old garden at Gawthorpe Hall, Lancashire Lancashire Council / SWNS.com Historic sites across UK exposed by heatwave Garden at Gawthorpe Hall, Lancashire which was taken out in the 1940's but can now be seen again through the parched grass Lancashire Council / SWNS.com Historic sites across UK exposed by heatwave Remnants of WWII Air raid shelters on Jesus Green in Cambridge show up in the parched grass SWNS.com Historic sites across UK exposed by heatwave Old crop marks in Norfolk show up Mike Page / SWNS.com Historic sites across UK exposed by heatwave Info showing how crop marks are made SWNS.com Historic sites across UK exposed by heatwave Old parts of RAF Lasham's airfield show up in the parched grass in Hampshire Jordan Bridge / SWNS.com Historic sites across UK exposed by heatwave Crop marks near Langstone, Newport RCAHMW / SWNS.com Historic sites across UK exposed by heatwave Crop marks show in Tregaron, Wales RCAHMW/SWNS Historic sites across UK exposed by heatwave Remnants of Caistor Roman town in Norfolk Mike Page / SWNS.com Historic sites across UK exposed by heatwave Penllwyn Roman fort in Ceregigion, Wales RCAHMW / SWNS.com Historic sites across UK exposed by heatwave Crop marks show in Tregaron, Wales RCAHMW / SWNS.com Historic sites across UK exposed by heatwave Old parts of RAF Lasham's airfield Jordan Bridge / SWNS.com Historic sites across UK exposed by heatwave Roman site near Magor, South Wales RCAHMW / SWNS.com Historic sites across UK exposed by heatwave Old crop marks in Norfolk Mike Page / SWNS.com Historic sites across UK exposed by heatwave Cross Oak hill fort in Talybont, Wales RCAHMW / SWNS.com

With Britain having experienced its longest heatwave since 1976 and the hottest June on record, a remarkable number of crop marks have appeared, sometimes showing the presence of buried remains that have lain unnoticed for decades or, in the case of the new discoveries, for centuries.

Newly discovered crop marks showing the outline of a prehistoric or Roman farm near Langstone, Newport, south Wales (RCAHMW/SWNS) (RCAHMW / SWNS.com)

In hot weather, crop marks expose the presence of things like historic buildings or burial mounds because the grass or crops above them grow differently compared to plants in the surrounding soil.

A buried ditch that once ringed a hill fort will collect more underground water and nutrients, so the plants above it will grow taller and greener while the hot weather parches all the neighbouring vegetation. The underground remains of a wall, meanwhile, will have drier and poorer soil above it, producing shorter and browner plants.

Ms Barker explained that the differences are more noticeable in the kind of prolonged hot weather that can affect even normally hardy greenery like grass. Which means that when the rain comes, the crop marks vanish again.

“It’s a frantic race against time and the weather,” said Ms Barker. “There is just this window of opportunity, and then, according to the forecast, in the next couple of days the rain will come and it will all be lost again – the outlines will start disappearing before your eyes.”

But even though the rain is coming, Ms Barker said the discoveries already made would produce great advances in archaeological knowledge, once the aerial images had been examined and the newly discovered features had been investigated at ground level.

A Roman fortlet whose remains have lain hidden beneath a field near Magor, south Wales, for centuries (RCAHMW/SWNS) (RCAHMW / SWNS.com)

She said: “We have found a Roman fortlet which will allow us to learn more about where the garrisons were and how the Roman conquest progressed. We have found an early medieval cemetery from maybe the eighth or ninth century, with the potential to tell us more about the period that used to be called the Dark Ages.

“At the known sites, we have seen more and more things come out as it has got drier and drier.”

Elsewhere in the UK, emerging crop marks have given archaeologists another look at the outlines of Venta Icenorum, the Romano-British predecessor of Norwich, which was first discovered when the RAF aeroplanes flew over it in the summer of 1928.

The ‘ghost garden’ of Gawthorpe Hall (Lancashire Council/SWNS) (Lancashire Council / SWNS.com)

The heat has also allowed the outlines of a “ghost garden” to emerge at Gawthorpe Hall near Burnley, Lancashire.

Designed by Sir Charles Barry in the 1850s, the garden was covered up after the Second World War. Now the exceptionally hot weather has allowed National Trust staff to see previously unnoticed features that may actually be from gardens that predated the work of Sir Charles, at a site which has been built on since the 14th century.

Ms Barker said: “The heat has allowed us to do things over a huge area and in a time frame that no amount of digging or ground radar surveys could have achieved, unless you had an unlimited budget and were willing to go slightly mad prospecting in hundreds of farmers’ fields on a hunch or nothing.

“I think it will add a huge amount to our knowledge.”

The heatwave has also allowed the public to discover their own crop marks revealing everything from wartime Anderson shelters to Bronze Age cremation sites.

The result has been a widespread sharing of discoveries on social media.