Power Bomb

Power Bombs are one of Samus' most powerful weapons. They must be deployed via Morph Ball mode; a few seconds after being deployed, they detonate, destroying most enemies within a surprisingly large radius. In Metroid Fusion, the blast also appears to cause a vacuum that can draw ordinary X Parasites (including ones that came from enemies killed by the blast itself) to the center of the blast.

In the Metroid Series
In Metroid: Zero Mission and Super Metroid, Power Bombs are gained from Power Bomb Expansions.

In Metroid Fusion, the Galactic Federation sends Samus the Power Bombs via Data Room.

In Zero Mission and Super Metroid, the Power Bombs are capable of opening Yellow Doors and breaking certain glass tubes, and in Fusion, they can destroy orange Bio-Barriers. In all three games, they are also capable of destroying Power Bomb Blocks.

In both Fusion and Zero Mission, Power Bombs are capable of revealing hidden passages and special blocks during the bomb's duration. In Super Metroid, this function is carried on to the X-Ray Scope.

In Super Metroid, Samus can perform the Crystal Flash, a technique that allows her to absorb the destructive force of the Power Bomb and convert it into energy to heal herself. She is only able to do this at critically low health levels, and it consumes a lot of her ammunition in the conversion process. She can also create Special Charge Beam Attacks using 1 Power Bomb and the Charge Beam.

The Power Bomb in Metroid: Other M is arguably its most powerful appearance. It plays a role in the training sequence at the beginning of the game when the Quarantine Officer instructs Samus to use her Power Bombs. In this game, it serves as a charged Bomb, and can be released when the 1 button is held for a set period of time.

In the Prime Series
In Metroid Prime and Metroid Prime 2: Echoes, the Power Bombs are arguably the strongest weapon in the game. They are able to destroy Yellow Blast Shields. It is also one of the three upgrades in Metroid Prime Pinball, where it significantly harms all enemies on one screen.

Metroid Prime
In Metroid Prime Samus had to fight the Cloaked Drone and get through a maze of electric barriers in the Phazon Mines to acquire Power Bombs; alternatively, Samus could obtain a Power Bomb Expansion in Phendrana Drifts before this by Sequence Breaking. Power Bombs can destroy objects made of Bendezium. If Samus dies while in Morph Ball form, what appears to be a Power Bomb detonates just before the game ends. This does not happen in Echoes or Corruption.

Metroid Prime 2: Echoes
In Metroid Prime 2: Echoes the Power Bomb Guardian in Dark Torvus must be defeated to acquire the Power Bombs. Samus can reach this guardian after she acquires the Spider Ball in the Sanctuary Temple. Power Bombs can destroy objects made of Denzium.

Metroid Prime 3: Corruption
Strangely, Power Bombs are not present in Metroid Prime 3: Corruption. However, a similar, super-powerful Morph Ball upgrade, the Hyper Ball, is acquired after defeating Mogenar on Bryyo.

Super Metroid manual
"The blast of these powerful bombs can hit everything on the screen. Blocks with the Power Bomb icon can be destroyed by Power Bombs."

Metroid Fusion manual
"This is the bomb in its most powerful form. Its explosion encompasses the entire screen, damaging a limited number of enemies. While in Morph Ball mode, press and hold the R Button then press the B Button."

Metroid Zero Mission manual
"This is the most powerful form of Bomb. Power Bomb explosions are massive, encompassing and damaging everything in view. Samus has a limited supply of these Bombs. To use them, hold the R Button while in Morph Ball form and press the B Button."

Metroid Prime 2: Echoes Inventory
"The Power Bomb is the strongest Morph Ball weapon. Press Y when in Morph Ball mode to drop a Power Bomb."

Samus's Notes: "Power Bombs do not have unlimited Ammo. Use them wisely. The Power Bomb can destroy many materials, including Denzium. Each Power Bomb Expansion you find will increase the number of power Bombs you can carry by 1."

Metroid: Other M On-Screen Tutorial
POWER BOMB Press A to change into Morph Ball mode, then press and hold 1 and release when fully charged.