Stonehenge, the mysterious Neolithic monument located in modern-day Wiltshire, England, has been a source of fascination for archaeologists for hundreds of years. The culture, or cultures, that constructed the site’s circles of stone monoliths between 8,000 and 2,000 BC left no written record as to its purpose, though the wealth of human remains suggests that it was a spiritual monument.

And while we may never know why Stonehenge was built, scientists continue to wonder how. Until recently, researchers had speculated that the site's largest towering stones, called sarsens, were cut from deposits in the Malborough Downs located about 30 kilometers (18 miles) away. Then, by unknown means, they were transported by the primitive builders to their current location.

But new findings by Stonehenge expert Mike Pitts, published in British Archaeology, suggests that several of Stonehenge’s most famous monoliths have actually been in the same location for millions of years.

Back in the 1970s, Pitt was allowed to excavate the earth around the largest sarsen, the “Heel stone”, which famously marks the location of the rising Sun on midsummer’s day. During this investigation, Pitts discovered evidence of a large, 6-meter (20-foot) diameter hole that had been created then filled back in.

The pit was too wide to have been dug to support a standing sarsen, but it is the right size to have contained the 5.9-meter (19.4-feet) tall, 35-ton Heel stone, once upon a time.

Unlike many of the other sarsens, the Heel stone stands away from the main circle of stones and has not been carved into a more uniform shape or altered to be attached to other stones. 

“If you are going to move something that large you would have to dress it before you move it to get rid of some of the bulk,” Pitts told The Times about his theory. "That suggests it has not been moved very far."

Pitts' team found another filled-in cavity next to a large monolith referred to as stone 16, located in the monument’s circle. Like the Heel stone, stone 16 is irregularly shaped, very large, and lines up with the path of the sun.

All of Stonehenge’s towering sarsens are composed of sandstone – rock made from sand, gravel, and bound with a silica cement – that formed on the land’s surface between 23 and 2.6 million years ago. Because large chunks of this type of sandstone are now uncommon on the surface of the Salisbury Plain area, geologists examining Stonehenge in the past believed that the rocks could not have been sourced locally.

Yet Pitts was able to back up his 1970s findings by drawing upon more recent geological investigations suggesting that sarsen sandstone is still present in significant abundance around Stonehenge, it has simply sunk into the ground following repeated cycles of soil freezing and thawing during ice ages.

"It’s possible that at the end of the ice age we had two really large visible sarsen boulders," said Pitts. And conveniently for astronomy-oriented ancient humans, they were "close together on the midsummer sunrise-midwinter sunset axis.”