Starting about 7,000 years ago, ancient humans in what is now northeastern Spain buried their dead deep in a cave, creating a necropolis of sorts that spans about four millennia and now contains more than 7,000 bones, according to archaeologists. And there are signs it may have been used for tens of thousands of years before that.

The Cova dels Xaragalls (Cave of the Ravines) was “a collective burial place,” archaeologist Antonio Rodríguez-Hidalgo, a researcher at the Catalan Institute of Human Paleoecology and Social Evolution (IPHES) and the Merida Institute of Archaeology, told Live Science in an email.

He said people were buried in communal graves within the cave starting about 7,000 years ago, during the late Neolithic or New Stone Age, though most of the Chalcolithic (“Copper-Stone”) period and throughout the Bronze Age, which ended in Spain about 3,000 years ago.