The 15,000-square-mile Ennedi Massif, in northeastern Chad, is a plateau the size of Switzerland. Between 350 million and 500 million years ago, this part of the globe was ocean. Then the ocean disappeared, leaving the sandstone floor exposed. The climate shifted from rain-soaked to arid. Sun, wind and water sculpted the sandstone into a dramatic, desolate and unearthly landscape of gorges and valleys, inselbergs and stacks, towering tassili and natural arches.