From the Super Mario Wiki
Brinstar Depths is a stage in Super Smash Bros. Melee based on a location in the game Metroid. As the name suggests, it takes place underneath Brinstar and is set on a large floating rock-type platform, surrounded by smaller rocks. There is magma far below the fighting stage, out of which the monstrous Kraid will rise. Kraid, who has a cameo in Nintendo Monopoly, is a large, dragon-like monstrosity and a nemesis of bounty hunter Samus Aran. He remains in the background and slashes at the platform, making it turn, forcing players to adjust their positions accordingly. Rarely, Kraid will hit a player when he swipes at the platform, but usually, the only danger comes from the possibility of falling off the edge.