Atlas Obscura
Umm Jirsan Cave in Khaybar, Saudi Arabia
Located in the Harrat Khaybar lava field in Saudi Arabia, Umm Jirsan is a 1500-meter-long lava tube system, with tunnels that are as much as 12 meters tall and 45 meters wide, and is estimated to have been formed nearly 3 million years ago. Lava tubes form when cooling rivers of lava leave behind large openings in the earth's crust. It had previously been suspected that early humans might have…