Engineers are completing repairs to a hole that appeared 2 years ago on the top of Silbury Hill in the midst of crop circle country, and hope to unlock some of its mysteries. English Heritage, which oversees the site, says the ancient man-made hill in Wiltshire has some kind of religious significance and is part of a group of ceremonial monuments that cluster around the village of Avebury. It is the largest artificial prehistoric mound in Western Europe, and measures 112 feet high and 550 feet wide, tapering to 92 feet wide at the summit.

Radiocarbon dating indicates it was built in several phases between 2800 and 2000 BC. The giant mound would have taken 700 men 10 years to complete, using antler picks and shovels made from the shoulder bones of animals.

The engineers are shoring up a collapsed 18th-century shaft sunk through the top of the hill. They will be able to determine the extent of the shaft and other tunnels and holes dug over the last 200 years. In 1776-7 the Duke of Northumberland used miners to sink the vertical shaft, which has now collapsed in the middle of the hill. They reportedly found no treasure inside.

Archaeologists will conduct a seismic survey, similar to a CAT scan, and create a three-dimensional internal map of the structure. Engineers will drill four small vertical holes down into the hill for the seismic scan, which English Heritage will use to plan their repairs of the monument. Drilling should finish by the end of September and the results will be known by October. The shaft has been temporarily filled with styrofoam blocks to make the monument safe for the survey.

Old aerial photographs show the hole on the top of the hill has been opened at least twice before, in the 1920s and 1930s. Despite a number of other excavations, no one has ever discovered the reason for building the enormous monument.

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