If you’re lucky enough to be one of the thousand or so tourists allowed into Upper Mustang each year, spring and autumn are the best times for trekking through this incredibly dramatic landscape. Tucked into a remote region of Nepal, it’s geographically part of the Tibetan Plateau. The area is surrounded by the Annapurna and Dhaulagiri Massifs and marked with deep gorges, terraced fields, and colorfully stratified rock formations. But perhaps most fascinating of all are the sky caves, 10,000 or so caves dug into the sheer walls of valleys in several areas of the district. Some of these man-made caves are thought to be ancient burial chambers; others are richly decorated with valuable Buddhist paintings, sculptures, and artifacts from the 8th to the 14th centuries. Perched over 150 feet above the valley floor, no one is entirely sure how the caves were built, what they were used for, or even how people accessed them.