Clouds encircle the lofty rim of Africa's Mount Elgon, a huge, long-extinct volcano on the border between Uganda and Kenya. The solitary volcano has one of the world's largest intact calderas, a cauldron-like central depression. The caldera is about 6.5 kilometers (4 miles) across and formed following an eruption, when the emptied magma chamber collapsed under the weight of volcanic rock above it.