The secrets of Britain’s most enigmatic prehistoric mound lie behind a locked green door at its foot. Yesterday the steel door was opened for the first time in 40 years to allow archaeologists access to the heart of Silbury Hill.

The 40m (131ft) hill, not far from the Avebury stone circle in Wiltshire, was built 4,500 years ago from 300,000 cubic yards (250,000 cubic metres) of chalk, with nothing more than hand tools and shovels made from deer antlers. No one knows why it was built, why the site was chosen or what its significance might have been.

English Heritage gave the go-ahead to a £500,000 attempt to unlock the secrets of Silbury Hill when it became clear that part of the largest man-made prehistoric monument in Europe was in danger of collapse.

In 1968 a televised attempt to reach its core by boring an 87-yard (80m) tunnel through the chalk was one of the BBC’s most ambitious outside broadcast projects, hosted by Magnus Magnusson.

Millions watched as the team of archaeologists, led by Professor Richard Atkinson of Cardiff University, approached the centre, where local legend said a great king in golden armour had been buried astride a golden horse. When they got there they found almost nothing.

The result was considered so disappointing by the BBC that it cut short transmission and the tunnel was backfilled as cheaply as possible by local builders using hardcore from an abandoned road project. With the funding gone, much of the follow-up work, including examination of the few finds that had been made, was never completed.

Since then subsidence has damaged the hill’s previously perfect profile. In 2000 a shaft sunk in 1776 collapsed, leaving a hole at the hill’s summit.

With essential work urgently needed, archaelogists were given the go-ahead to reenter Silbury Hill.

As workmen cleared away the turf and rubble covering the tunnel entrance yesterday, excitement rose. The concrete lintel had the date 1968 incised into it. A brush-down of the steel door revealed a large stylised “S” for Silbury, the logo used by the BBC for its series.

The key to the rusty padlock was discovered “under the door-mat” at its base. Local pagans began a chant, and Archdruid Terry Dobney cradled a lump of chalk removed from the centre of the mound that he wanted to return.

Behind the door was a wall of loosely packed rubble that will now be removed as diggers make their way inside the hill.

Mike Pitts, editor of British Archaeology, said that the new investigation promised to reveal secrets not accessible to previous excavators. As a 15-year-old schoolboy he had watched the original 1968 dig.

“The BBC made the mistake of expecting something spectacular to come out of it,” he said. “When all they got was a bunch of archaeologists looking at a patch of decayed grass they pulled the plug. It’s remarkable how little we know about the 1968 dig, as the work was never written up.

“Beneath this mound is what is probably the best-preserved prehistoric landscape of this kind in Europe.”

Environmental archaeology, the study of soil, plants and insect remains, had advanced dramatically since the last time the door was opened, said Dr Robert Bewley, regional director of English Heritage. “We will be able to get much more precise dates than were possible then,” he said. “There is also a lot that we still don’t know, such as whether the mound was built in one go or was the work of many generations.

“This will probably be the best chance this generation will get to find those answers.”

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