New excavations are revealing the truth behind Silbury
It is thought the stairway at the 4,500-year-old site was used for ceremonial purposes.
Six months ago, archaeologists from English Heritage began a modern-day investigation into the monument's intruguing past.
First had to fill a collapsed mine shaft sunk in the 18th Century, to protect the monument from further damage.
Spiral fashion
A 3D seismic survey has revealed what could have been under the mysterious chalk mound.
A spokesman said it was possible that it was 31 metres high and built in a spiral fashion with a path for ceremonial processions.
Digital pictures of the survey will soon be available on the internet.
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.
"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."