Missile Tank

From Wikitroid, the Metroid wiki

A Missile Tank or Missile Expansion is a tank that allows Samus Aran to fit more Missiles into her Missile Launcher. They are often hidden at the end of secret passages. Usually the first one is found out in the open, however. Missiles are always either the first or second item Samus acquires, sometimes switching position with the Morph Ball. They are found in every Metroid game.