IT was 25 years ago that a tourist visiting Wiltshire from Germany made an astonishing discovery at Silbury Hill.

A 14-metre-deep crater opened up on the summit of the largest prehistoric chalk built mound in Europe on May 29, 2000 – an echo of efforts by archaeologists and antiquarians in days past to try and get to the bottom of its secrets.

Found near Avebury in Wiltshire, Silbury Hill is steeped in mystery as experts have been baffled over its intended use.

English Heritage states that Silbury Hill was built between 2470 and 2350 BC and stands at a little over 30 metres high and 160 metres wide at its base – making it close in height and volume to contemporary Egyptian pyramids.

This giant prehistoric built mound, which sits alongside the Bath Road between Beckhampton and Marlborough, had been closed to the public for a quarter of a century.

Erosion of irreplaceable archaeology from footfall and Site of Special Scientific Interest (SSSI) status did not deter climbers from tackling the enormous chalk-built wonder of the Avebury World Heritage Site.

This was in part the problem following the collapse.

Brian Edwards, a visiting research fellow at the Regional History Centre at the University of the West of England, has revealed the behind-the-scenes details of the day that the collapse was first discovered.

“As darkness approached on May 29, 2000, all was quiet at Silbury,” he said.

“A German tourist had encountered a ragged hole close to the centre of the summit of the mound, which prompted the site to be searched and then cordoned off and made safe by Dr Ros Cleal, curator of the National Trust’s Alexander Keiller Museum at Avebury, and her late husband Chris Gingell, who was the National Trust’s Avebury property manager.

It took English Heritage a decade and over a £1million to repair the shaft and tunnels Picture: Brian Edwards

“Chris then spent the night on the Hill to safeguard any inquisitive members of the public.

“The following morning, Amanda Chadburn, inspector of Ancient Monuments for English Heritage, received the news from Silbury with a little disbelief; how could Silbury Hill collapse?”

A local newspaper reporter Nigel Kerton received a tip-off about the collapse and lowered a digital camera into the void, with the resulting image resembling a square bottomless lift shaft.

The shaft that was discovered was eight feet square and was first mentioned in regional newspapers in November 1776.

It is believed that antiquarians and archaeologists were spurred on by the inherent mystery of this site, as at least three separate tunnels were dug into the centre of the hill.

Brian said: “Newspapers reported in November 1776 that Mendip miners had begun to open up Silbury Hill. ”

‘They have made a hole at the top of eight feet square. The Antiquarians promise to themselves wonders from the bowels of his mountain!’

“The organiser that recruited those miners in 1776 was the extremely rich Edward Drax of Charborough Park in Dorset.”

Accompanied by his wife, 33-year-old Mary Churchill, and their daughter Sarah, 7, the 50-year-old Drax habitually spent the Bath season living on Queen’s Parade and was associated with the entertainments at the New Assembly Rooms and the weekly gatherings nearby at Mrs Miller’s Batheaston Villa.

Brian added: “Just a week after his arrival being announced in September 1776, an advertisement on the front page of the Bath Chronicle invited ‘The Curious in Antiquities’ to subscribe to an excavation of Silbury Hill – ‘there is no account of its having ever yet been searched into’.

Brian said that one of the most persistent myths of Silbury Hill is that the treasure-hunting Duke of Northumberland, Hugh Percy, led this first of a series of Silbury’s excavations across three centuries that found nothing at all.

“It is not true,” Brian said. “The Duke was merely an enthusiastic subscriber; the excavation sought knowledge, and of many misunderstood finds, the top-to-bottom Drax shaft encountered a tall pole around which Silbury had been built.”

Fast forward to May 29, 2000, and it was apparent that material within the 1776 top-to-bottom shaft had somehow collapsed.

Brian added: “It was not recognised, let alone understood, 25 years ago. But it is likely that this 18th-century shaft was never entirely backfilled.

“In time the contents began to settle, then further slump each time there was a heavy downpour.

“In 1849, the contents of the shaft were undermined by a tunnel excavated from the side of the hill by the Archaeological Institute.

“Zealously led by Dean John Merewether, this exploratory tunnel reached the centre of Silbury.

“The money ran out, but fliers circulated which mentioned the prospect of encountering gold – this is the origin of the myth of a life-size gold statue of King Sil on a horse.

“Merewether then carved out a semi-circular route with exploratory offshoots within the middle of the mound until it also connected with the vertical shaft.

Silbury Hill as it looks now Picture: English Heritage

“Material started to fall from the Georgian shaft, and evidently the contents continued to empty into the Victorian tunnel, which was abandoned and only the entrance sealed.

“Recorded in drawings made in 1849, a depression in the summit was evidence of the continuing settlement of the material in the shaft.

“This dip in the middle of the summit was evidently topped up from time to time, but without any record being made.

“It still existed in the beginning of the 20th century, at which time the entrance to the 1849 tunnel also collapsed, and pupils from Marlborough College were known to be crawling into the void despite the tunnel roof being prone to regular collapses.

“Between 1968 and 1970 the BBC excavated a new tunnel to connect with the tunnel of 1849. This BBC tunnel was not backfilled either.

“It took English Heritage a decade and over £1 million to repair the shaft and tunnels. There is now no sign anything ever happened.”

It is estimated that construction of Silbury Hill would have involved 4 million man hours of work and would have likely been enlarged over several generations.

Half a million tonnes of material, mostly chalk, were used to create it.

English Heritage commissioned a major programme of conservation work to stabilise Silbury Hill in 2007.

This project provided a unique opportunity for archaeologists to find out more about the monument.

The 1968 tunnel was reopened, giving archaeologists a final opportunity to record the inside of Silbury Hill. Environmental samples were taken from archaeological deposits, and a remote-controlled filming vehicle was used to record inaccessible areas. Excavations also took place on the summit.

All known voids inside the hill and the crater on the summit were then refilled with chalk, using 1,465 tonnes of material.

The mound has now been restored to as near its original condition as possible.