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“A rich garden of life has grown on the megaliths, an exceptional lichen garden has grown. So it’s potentially quite concerning.”

The future host of TV shows such as PBS’ “Nova ScienceNow” was part of an expedition led by Gerald Hawkins, the scientist who first theorized that Stonehenge’s mysterious megaliths were an ancient astronomical observatory.

“With these megaliths, it’s the end of the era of these big sites with people being killed.”

"This differs from what we usually see in megalith graves, i.e. stone burial chambers from the Neolithic period," Karl-Göran Sjögren explains.

Mr Clarke has also dated some of the rocks that became the megaliths of Stonehenge.