Majlis al Jinn Cave, Oman

Photograph by Stephen Alvarez, National Geographic

An explorer descends into the Majlis al Jinn cave in Oman. At more than 50 stories deep, it's one of the largest caves in the world. The cavers' mission is to determine if Oman's deep caverns could be safe for tourists to enter.

(For more pictures of the amazing colors of our world, buy the National Geographic book Life in Color.)