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The 'Great Dark Spot' was a dark spot on Neptune similar in appearance to Jupiter's Great Red Spot. Although it looked nearly the same as Jupiter's spot, it was not thought to be a storm but instead an atmospheric hole similar to the hole in Earth's ozone layer.
The spot was detected by NASA's ''Voyager 2'' probe, which passed Neptune in 1989. It was comparable in size to Jupiter's spot, and was located in Neptune's southern hemisphere. Winds measured in the spot traveled at speeds up to 2400 km/h (1500 mph), the highest of any planet. The spot appeared to change as the spacecraft flew by, and does so in many pictures of it. The spot was believed to have been rotating counterclockwise. Concentrated regions of crystal methane and frozen water particle clouds resembling Earth's cirrus clouds were discovered over the Great Dark Spot.
When the Hubble Space Telescope viewed Neptune again in June 1994, the spot had vanished. However, another spot very much like the old one appeared in the planet's northern hemisphere later that year. ^[1] It is not known why the Great Dark Spot appeared nor whether it is a common occurrence or a rarer phenomenon. Its atmosphere is mainly hydrogen and helium. Some scientists think of the Great Dark Spot moves because of the 2250 km/h (1400 mph) jet stream winds on Neptune.

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References

1. "Views of the Solar System: Neptune"