The Great Dark Spot was a dark spot on Neptune similiar to Jupiter's Great Red Spot. Although it looked the same, it was not a storm, like Jupiter's spot, but was thought to be a hole similar to the hole in Earth's ozone layer.
The spot was detected by NASA's Voyager 2 probe, which ventured to Neptune. It was comparable in size to Jupiter's spot, was located in the southern hemisphere, and winds were measured in it at up to 1,500 miles an hour (the highest of any planet). The spot appeared to change as the spacecraft flew by, and does so in many pictures of it.
When the Hubble Space Telescope viewed Neptune again in 1994, the spot had vanished. However, another spot very much like the old one appeared in the planet's northern hemisphere.
