British researchers have discovered that a famous 5,000-year-old burial mound in Wales is actually just the outward part of a much larger funerary complex. The discovery was possible using ground-penetrating radar survey and 3D modeling that showed there’s a lot that lies beneath the soil.

The site, Bryn Celli Ddu (the Mound of Dark Grove in Welsh), is located in Llanddaniel Fab on the Welsh island of Anglesey. The mound is one of the most intriguing prehistoric monuments in Britain. It was first excavated and reconstructed in 1928 and was designed to be aligned with the rising Sun during the summer solstice.

“We know that Bryn Celli Ddu sits in a much more complicated landscape than previously thought," Dr Seren Griffiths, from the University of Central Lancashire who works on the site, said in a statement. "Over the last three years, we have discovered 10 new rock art panels, and this year the picture has developed to include further evidence for a new Bronze Age cairn along with a cluster of prehistoric pits.”

“We have evidence for over 5,000 years’ worth of human activity in the landscape, ranging from worked flint derived from the tool making efforts of our prehistoric ancestors to prehistoric burial cairns and pits with pottery deposited within,” she added.

The archaeological research has taken place over the last three summers, and this year lasted from June 11 to June 24. During this time, the team discovered a series of circular anomalies just behind Bryn Celli Ddu. What they discovered was Late Neolithic to Early Bronze Age cairns, piles of stones placed above burial pits.

“The geophysical survey, combined with innovative digital modeling, has raised the possibility of a cairn cemetery surrounding the original monument, something never suspected before,” Dr Ben Edwards from Manchester Metropolitan University, explained.

The project is completely public and people can go visit the site as the researchers investigate the mound. On June 17, an incredible 750 members of the general public visited Bryn Celli Ddu and took a tour of the open trenches.

“Since we started the project we have discovered that Bryn Celli Ddu was never in isolation, there was activity happening all around,” added Dr Ffion Reynolds from Cadw, the historic environment service for the Welsh Government.

“We knew this would be a good project but it’s turning out to be very exciting.”