Underneath the ground just two miles away from Stonehenge lies an ancient stone monument consisting of at least 90 large stones. The stone monument formed part of the border of Durington Walls, otherwise known as "superhenge," The Guardian reports.

Scientists believe the monument is some kind of a Neolithic ritual site. Its stones, some of which measure up to 4.5 meters, are the largest that have ever been discovered.

"What we are starting to see is the largest surviving stone monument, preserved underneath a bank, that has ever been discovered in Britain and possibly in Europe," Bradford University archaeologist Vince Gaffney, head of the Stonehenge Hidden Landscapes Project, told The Guardian. "This is archaeology on steroids."

The underground row of stones was discovered through the Stonehenge Hidden Landscapes Project, a five-year project that aims to create an underground map of the area. The researchers used remote sensing and geophysical imaging technology that revealed the presence of the stones without excavation, BBC reports.

"Our high-resolution ground penetrating radar data has revealed an amazing row of up to 90 standing stones, a number of which have survived after being pushed over, and a massive bank placed over the stones," Wolfgang Neubauer, director of the Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology, told CNN.

The stones are believed to be 4,500 years old. Neubauer said 30 stones have remained intact while others are fragmentary.

The discovery of the underground stone monument has raised new questions about Stonehenge and those who built it.

"The extraordinary scale, detail and novelty of the evidence produced by the Stonehenge Hidden Landscapes Project, which the new discoveries at Durrington Walls exemplify, is changing fundamentally our understanding of Stonehenge and the world around it," University of Birmingham archaeologist Paul Garwood told CNN. "Everything written previously about the Stonehenge landscape and the ancient monuments within it will need to be rewritten."