Boulders, also known as metal balls, are rare obstacle in Super Metroid. Encountered only in three secret areas, they rest on unstable ground and immediately begin rolling towards Samus Aran. Since they are 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.
In Maridia, there are two secret rooms only accessible by sinking into quicksand pits. In addition to several expansions, these two chambers also contain boulders hidden in the ceiling that fall onto Samus if she steps on the floor directly underneath them.
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.
In Metroid Prime: Federation Force, boulders appear towards the end of M02: Hardball on Bion. They are launched down a steep slope, and bounced off of platforms under which the Federation Force can take cover in order to avoid them. It is possible that this is a trap set by the ancient Bion race to protect its Orb, the disturbing of which is what triggers the boulder avalanche. Unlike in Super Metroid, these boulders can be destroyed quite easily.
They also appear in M20: Insurrection, after navigating a maze of Thermo Spheres while out of their Mechs. The Federation Force drops into a sloped chamber where an infinite number of boulders roll down, and the leader's Mech is present. This enables the leader to shoot as many boulders as possible to protect any fellow soldiers in the mission (whose Mechs are not found until later).
- Page 51
- "Keep an eye open for the boulders rolling from ledge to ledge and jump them when necessary."
- IRON AVALANCHE
- "Watch out for the metal balls that tumble down the slopes. Simply jump over them."