In 1989, Voyager 2 flew by Neptune. It discovered a huge dark spot on the southern hemisphere. The Great Dark Spot was an anti-cyclone about the size of Earth. It circled the planet every 18.3 hours. Further south, the space probe found a formation of white clouds and a smaller dark spot. The smaller dark spot circled the planet in 16 hours. The formation of white clouds, called the Scooter, circled in 16.8 hours. The Scooter appears to be a storm that moves at a speed of about 1,800 miles per hour (2,900 kph).
A new theory suggests that the dark spots on Neptune may not be cyclones and anti-cyclones. Instead, they may be holes in the blue methane clouds. They may have been opened up by high-pressure systems, showing the darker ammonia and sulfur clouds below.
