According to a new report by Science Advances, the massive stones at Stonehenge in Southern England came from a region 15.5 miles away from the historic site. The sandstone boulders, or sarsens, were moved from the West Woods in the English county of Wiltshire. The sarsens measure up to 30 feet tall and weigh roughly around 25 tons.

“Until recently we did not know it was possible to provenance a stone like sarsen,” said David Nash, the study’s lead author, in a statement. “It has been really exciting to use 21st century science to understand the Neolithic past and answer a question that archaeologists have been debating for centuries.”

Nash asserts that the stones were transported down the Wiltshire Avon Valley to the east or through a western route across Salisbury Plain.Researchers used a portable X-ray fluorescence spectrometry to examine the stones. “Next, the researchers performed inductively coupled plasma mass spectrometry (ICP-MS) and ICP-atomic emission spectrometry (ICP-AES) of samples from a core previously drilled through one sarsen stone and a range of sarsen boulders from across southern Britain,” the scientists explained, in the statement.

After carefully analyzing the sarsens, the team discovered that they originated from West Woods. “The reason the monument’s builders selected this site remains a mystery, although the researchers suggest the size and quality of West Woods’ stones, and the ease with which the builders could access them, may have factored into the decision.”

There are two different types of stones erected at Stonehenge. The sarsens are the bigger silica stones in Stonehenge’s outer ring and center. There are currently 52 stones on the site today, but the researchers suggest that there were actually 80.