Anxious experts survey Silbury

A seismic survey of Silbury Hill is to be carried out to reveal the pits and tunnels that threaten Wiltshire's giant enigmatic neolithic monument.

Archaeologists concerned by the prospect of Europe's largest manmade mound falling apart are backing the plan, despite some anxiety over the delay before repairs can be made.

Meanwhile, the hill remains out of bounds to visitors after a partially refilled 225-year-old pit collapsed following heavy rains last autumn. It left a 12m deep hole at the summit.

English Heritage will repair the damage after a three-dimensional study of the hill's interior, scheduled for early July, is complete.

This will reveal the cavities left by treasure hunters and amateur archaeologists through the centuries.

Cavities will need to be dealt with if further collapse is to be avoided. It might also shed light on Silbury Hill's mysterious purpose.

An English Heritage spokeswoman said: "We're aware people are anxious and we are doing our best to sort it out as fast as possible."

Lord Renfrew, director of the McDonald Institute for Archaeological Research at Cambridge University and an opposition peer, said he was satisfied the plan was sound.

"Silbury Hill is one of the great monuments of the prehistoric world, Britain's equivalent of the pyramids, and it is alarming if the situation is getting worse," he said.

"However, there is no sense in just pouring concrete into holes until you know where the hole goes - the matter should be researched thoroughly."

Campaigners calling for more funding for archaeological investigation and conservation will be protesting at the foot of the hill on May 19.

Clare Slaney, one of the protest organisers, said the delays put the hill in jeopardy and that some archaeologists who sympathised with the demonstrators felt unable to express their opinion publicly.

"Fear of whistleblowing and losing their job means that, when the world is gasping at the Taliban destruction, no one knows anything about another World Heritage monument being destroyed through bureaucratic neglect," she said.

Terence Meaden, an archaeologist and retired professor of physics, declined an invitation to address the demonstration.

"Repairing the monument must be done properly - a protest will not help at all," he said.