Neptune is a stormy planet with the strongest winds in the Solar System. It also has areas of highly intense storms. These are shown in the atmosphere by dark spots. The biggest dark spot, known as the Great Dark Spot, was first observed in 1989 by Voyager 2 when it visited Neptune. This spot was around the same size of Earth and is similar to the Great Red Spot seen on Jupiter. The storms in the spots also force icy methane crystals high up into the atmosphere of Neptune, forming white clouds.
The Hubble Space Telescope, a telescope orbiting Earth to take pictures of deep space, took pictures of Neptune but found that the Great Dark Spot discovered by Voyager 2 had disappeared, and that a new dark spot had appeared somewhere else (shown below). Nobody is sure why this has happened, and why Neptune is able to lose its spots but Jupiter still has one which has been observed for hundreds of years.