Planet Zebes

From the Super Mario Wiki

Planet Zebes's appearance in Super Smash Bros..

Zebes is a planet seen in the Metroid series. It is the main setting of the first game in the series, Metroid. The surface is mostly barren and the underground is riddled with large caverns.

Planet Zebes also appears as a stage in Super Smash Bros.. It is a rather small stage, and it is easy to get blasted off of it. The only other feature is the acid, that rises up about every 30 Seconds. The acid will rise up, and damage any player who touches it. After a while, it will rise up so high that only the center platform, which is also the tallest, is safe. Samus is always fought here in 1-P Mode.

The stage itself wasn't put into the sequel, Super Smash Bros. Melee. It was replaced by a similar stage called Brinstar, which followed the same feature to it, but differed greatly in appearance.