Neptune
Voyager 2 visited Neptune on 25 August 1989. It was the only spacecraft to visit the planet. Neptune used to have a huge storm known as the "Great Dark Spot". Voyager 2 discovered the spot in 1989. The dark spot was not seen in 1994, but new spots were found since then. It is not known why the dark spot disappeared. Visits by other space probes have been planned.