From the Super Mario Wiki
Planet 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.. The only feature is the lava, that rises up about every 30 seconds. The lava rises 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.