A hole appeared in the summit last year

The alien hunters broke into the site under cover of darkness soon after the hole was discovered. Evidence of the trespass has only come to light now, after a video of the foray was included in a documentary film.

After tunnelling under the temporary roof installed by English Heritage, two climbers abseiled into the shaft, while a third videoed the stunt. They repeated the climb on a rainy winter night six months later.

Archaeologist Mike Pitts says the climbers dislodged the soft clay sides of the unstable shaft, possibly damaging vital archaeological clues.

"They were extremely lucky to come out again alive. Between their two visits there was a major collapse, which they joked about in the video of their December visit."

Silbury Hill is part of the Stonehenge circuit

Footage of the illicit descent has since been incorporated into a video by Dutch documentary makers Janet Ossebaard and Bert Janssen, who have attempted to sell it on the tourist circuit in nearby Avebury.

Mr Pitts says that not only have shopkeepers refused to stock the video, the crop circle enthusiasts who flock to Wiltshire every summer have almost universally condemned the stunt.

English Heritage itself has come under fire for not doing enough to protect the site.

"This video depicts the criminal trespass of an ancient monument. So how these people can imagine that they can sell it without being criticised is impossible to believe," says Mr Pitts.

Secrets of the mound

As repairs to the hill get under way, English Heritage is carrying out a seismic study to create a 3D computer image of what lies inside.

The mine shaft is slowly subsiding

It is the latest attempt to unlock the secrets of the hill, which forms part of the ancient landscape surrounding Avebury stone circle and nearby Stonehenge.

Theories abound as to why it was built. Many believe it was a sacred monument, while others think it may have been a Stone Age waste tip.

But the trespassers - thought to be local crop circle enthusiasts - apparently believe that Silbury Hill is a power station for passing alien craft.

It is a view shared by the Dutch documentary makers, who posted an account of the stunt - and assorted UFO sightings near Silbury Hill - on their website.

Included is a photo of a burn mark on a mobile phone carried by one of the climbers, apparently caused by intense heat on his climb up the mound - evidence, they infer, of the power emanating from the hill.

Mr Pitts is sceptical: "I suggest that they may have jumped on the phone while clambering about in the dark."