A hole appeared in the summit last year

The collapsed 18th Century shaft has already been filled in to protect the monument from further damage.

Dr Kevin Brown, regional director of English Heritage, said: "The results of the seismic survey are very encouraging, as they have shown that the hill's structure appears stable.

A team of experts will shortly return to the hill to gather more detailed information prior to preparing a full assessment of future maintenance. English Heritage archaeologist Fachtna McAvoy said: "We can see what is effectively a Neolithic building site at the base of the mound.

Silbury Hill is part of the Stonehenge circuit

"The workmen were evidently struggling with wet ground conditions and churned up the land surface into a mixed layer of chalk and mud.

"We have also discovered that the mound, when it was built, was 31 metres high and that there were no long layoff periods during its construction."