Archaeologists have been studying and digging the megaliths of Stonehenge since 1620s. But the World Heritage Site still holds a lot of secrets. Using high-tech methods, researchers have found a hidden complex of monuments at Stonehenge, challenging the long-standing belief that it stood in isolation, reports BBC. The monument is about 85 miles southwest of London and attracts more than a million visitors every year.

Stonehenge Not Alone! Hidden Monuments Found At The Site

Evidence of 17 previously unknown monuments found around Stonehenge

Results of  four-year long survey were revealed as part of the British Science Festival. Scientists used high-tech, non-invasive techniques such as high-resolution magnetometers and ground-penetrating radar to produce digital maps of what’s beneath the Stonehenge. They found evidence of a huge timber building and 17 previously unknown ritual monuments. The timber building is believed to have been used for burial ceremonies. Scientists estimate that it was 3,000 years older than Stonehenge.

Scientists also discovered some prehistoric pits, many of which seem to be aligned with the sun. Stonehenge was raised about 5,000 years ago. Project leader Vincent Gaffney, an archaeologist at the University of Birmingham, said Stonehenge is a ritual monument. But it’s not standing there by itself. It’s part of a very complex landscape with ritual activities that go around it, Gaffney told LiveScience.

Why was Stonehenge built?

Stonehenge is one of the most extensively studied historical monuments in the world. But archaeologists haven’t yet been able to figure out why it was built. Its ruins stand as a marvelous feat of prehistoric engineering. The biggest stones at the site weigh up to 25 tons and are about 30 feet tall. Gaffney said it would take his team at least one year to process all the data they gathered during the four-year survey.

And then the English Heritage, the government body that looks after historical and archaeological sites, will decide where to dig up in a traditional excavation. That should help reveal the age of these monuments, burial mounds and pits.