From the Super Mario Wiki
Brinstar Depths is a stage in Super Smash Bros. Melee and Super Smash Bros. Ultimate 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 Kraid, a large, dragon-like monstrosity and a nemesis of bounty hunter Samus Aran, will rise. He remains in the background and slashes at the platform, making it turn, forcing players to adjust their positions accordingly.
Similar to the Brinstar stage, a large brain can be seen, but not interacted with; in this stage, it is found inside a spherical cage covered in spikes embedded into the stage itself, and is much smaller than the other one.
Names in other languages