Some observers believe Stonehenge was once a full circle of enormous stones—and now there seems to be proof there are even more Neolithic monuments underneath it. A lot of them: Radar mapping that's peeked as far as 2 miles beneath the surface has uncovered an elaborate subterranean network of 17 monuments, plus evidence of more than 50 big stones that are around the same age as the above-ground versions, Gizmodo reports. The stones, which are being called a "super henge," are part of the already-excavated Durrington Walls dirt bank and are "a big prehistoric monument which we never knew anything about," one of the project's archaeologists tells Nature.
The discovery was made by researchers with the Stonehenge Hidden Landscape Project, which has been using geophysical info to "create a highly detailed archaeological map of the 'invisible' landscape." This includes the "Cursus," a 2-mile-long pit with sections that line up with the summer solstice sunrise and sunset, and the 1-mile-round "super henge," riddled with holes where 10-foot-long stones once sat—some are still there. Researchers still don't know what the place was, with Smithsonian Magazine suggesting everything from a temple or parliament to a graveyard. (Read about a Stonehenge mystery that may have finally been solved.)