They've puzzled the world for centuries, but scientists have finally solved mystery of the place of origin of many of the megaliths that make up Stonehenge.
And it's all because of a fragment of Stonehenge, taken as a souvenir, that had been sitting in the United States for decades.
Stonehenge sits in Wiltshire, England, and new geochemical testing indicates that 50 of Stonehenge's 52 pale-gray sandstone megaliths, known as sarsens, share a common origin about 15 mile away at a site called West Woods on the edge of Wiltshire's Marlborough Downs.
Timothy Darvill is a Professor of Archaeology at Bournemouth University.
"The science behind it is fairly straight forward in one sense because what we're doing is a simple case of finger-printing. We're taking some stones at Stonehenge itself and we're working out the geochemistry of them. For that we measure all the little trace elements which are in the stone. Now, sarsens are really difficult stone to work with because it's 99 percent silica and silica is a pretty ubiquitous mineral."
The American connection ion is over a sarsen core sample, extracted during conservation work in the late 1950s when metal rods were inserted to stabilize a cracked megalith, which now has provided crucial information.
It was given as a souvenir back then to a man named Robert Phillips who worked for the company involved in the conservation work and was on-site during drilling.
Phillips took it with him with permission when he emigrated to the United States in 1977.
Phillips decided to return it to Britain for research in 2018. He died this year.
Stonehenge is one of the UK's most recognised landmarks, but their exact purpose still remains unknown to scientists.