Stonehenge presents many mysteries, including, of course, who built the massive stone structure. That question aside, a great head scratcher remains why anyone would schlep stones weighing anywhere from 2 to 4 tons some 180 miles from quarries in Wales to the Salisbury Plain, instead of carving the massive blocks from local stone.

Between 2014 and 2016 archaeologists looked for an answer while excavating the two bluestone quarries in the Preseli hills of Wales at Carn Goedog and Craig Rhos-y-felin, where at least five of the 80 or so bluestone blocks at Stonehenge are believed to have originated.

There they found that the hard bluestone poking up from the ground was more or less vertical. That supported the idea that rock miners could have popped six-foot “ready-made” sections of the rock loose rather than having to carve them out of a more local source.

At those sites, they also found stone tools and charcoal mixed with dirt and stone in "loading bay" platforms created by stone workers. That suggests that after the bluestone was lowered onto platforms, a team of burly men could have then dragged the rock on sledges along a route that parallels the modern A40 highway. The research appears in the journal Antiquity.

Mike Parker Pearson, an archaeologist at University College London and lead author of the study, tells Ben Guarino at The Washington Post that construction crews at other monolith sites around Europe rarely used stone from more than 10 miles away, often knapping them on site. But the ready-made bluestone flecked with bits of white may have been worth the trip, with Pearson jokingly calling them “the Ikea version of Neolithic megaliths.”

The quarry site may also help archaeologists revise Stonehenge's timeline. According to a press release, the team was able to carbon-date some of the charcoal found among the platforms, showing that the quarry was in use around 3,000 B.C., which corresponds with the time construction at Stonehenge began.

Pearson and his colleagues now posit that Stonehenge was originally a circle of unworked bluestone, much of it quarried in Wales, that was set up in pits at the site now known as the Aubrey holes. It wasn’t until 500 years later that the iconic trilithons—the stone formations shaped like the Greek letter pi made from local sandstone, were erected at the site.

The finds at the quarry also tell a different story of how the big rocks made it to Salisbury. Esther Addley at The Guardian reports that researchers previously believed the bluestones were primarily transported to Stonehenge via water. “The idea was that they were dragged down the southern slopes to Milford Haven, and then transported by raft along the Severn estuary and along the River Avon to Salisbury plain,” says co-author Rob Ixer also of University College London. But the quarries show that the stones come from the northern side of the hills, not the south, which is closer to the ocean. And Guarino reports that attempts to float a similar-sized stone using Neolithic technology in 2000 failed spectacularly when the stone and its raft sunk. “Instead we now believe, and it seems pretty likely, that the stones were all manually transported,” Ixer tells Addley.

The fuzziness of the carbon dating timeline also leaves open the possibility that the bluestone circle was originally constructed in the Preseli hills. Researchers hope to examine other stone circles in the area to figure out if Stonehenge actually began in Wales before being dragged to Salisbury.

As to why the monument's builders dragged the heavy stones so far, it’s still difficult to say. Parker Pearson tells Guarino that Stonehenge was constructed at a time of economic and population decline among the people of Great Britain. Stonehenge could have been a community-building exercise. “A major event like this would have brought together disparate communities that were growing apart,” he says.

Ixer, however, tells Addley at The Guardian that though we are filling in the timeline, it’s not possible to fully decipher the mysterious stones. “We will never understand Stonehenge. That is the beauty of the monument,” he says.