Photograph courtesy NASA

Neptune's Great Dark Spot is clearly visible in this image of the small planet, taken in 1989 from Voyager 2. The Great Dark Spot was a rotating storm system similar in size to Jupiter's Great Red Spot. Winds near the spot were measured up to 1,500 miles an hour (2,400 kilometers an hour)—the strongest recorded on any planet. When the Hubble Space Telescope viewed Neptune in 1994, the storm system had vanished and another dark spot had cropped up in the planet's northern hemisphere.