The Great Dark Spot generated large white clouds at or just below the tropopause layer similar to high-altitude cirrus clouds found on Earth. Unlike the clouds on Earth, however, which are made of mostly crystals of ice, Neptune's clouds are made up of crystals of frozen methane. And while Neptune's clouds usually form and then disappear within a period of a few hours, the clouds in the Great Dark Spot were still present after 36 hours, or two rotations of the planet.