British scientists say they have solved a mystery involving Stonehenge, the world-famous stone monument in Wiltshire, England.

The scientists reported this week that they have identified where many of Stonehenge’s large stones, called megaliths, came from. The researchers said part of one megalith helped solve the mystery. The small piece of stone had been kept in the United States for over 40 years.

Chemical testing suggests that most of Stonehenge’s megaliths, known as sarsens, came from an area called West Woods. It is about 25 kilometers away from the ancient monument, the researchers said on Wednesday.

People set up the sarsens at Stonehenge over 4,000 years ago. The largest sarsen is 9.1 meters tall. The heaviest weighs about 30 tons.

Nash said that researchers still do not know how people moved the stones to Stonehenge. “Given the size of the stones, they must have either been dragged or moved on rollers,” he said. Nash added, “We don’t know the exact route but at least we now have a starting point and an endpoint.”

A sarsen core sample, taken during repair work in the late 1950s, gave important information about Stonehenge’s origins. The core sample was given to a man named Robert Phillips. Phillips worked for the company that was repairing the monument.

Nash said he hopes the finding will help people better understand the hard work that went into building Stonehenge.