Boulder

Large, rolling Boulders are a rare obstacle in Super Metroid. Encountered only in two secret areas, they rest on unstable ground and immediately begin rolling towards Samus. Impervious to her weaponry, the only way she can avoid them is by jumping over them. When they hit a wall, they explode into chunks. They reappear after leaving the room.

There are two places where Samus encounters rolling and falling Boulders. In Ridley's Lair, if Samus enters the room with the large monster-like cave, the upper shafts feature three Boulders. These guard a Missile Tank. If Samus lays a Power Bomb in the third room of the "old" Brinstar, it will obliterate the bottom half of the large overhang and reveal a shaft up which she can Shinespark. Crossing over into the next room, Samus will find a floor of spikes submerged in water with no apparent way over, although using the X-Ray Scope reveals a hidden platform above her. However, Boulders will fall and roll towards her. Beyond this room is a pair of Missile Tanks.