Bomb rocks are round, cracked rocks that are composed of explosive materials. They can explode and cause massive damage to walls, enemies, and pikmin.
The bomb rock was introduced in the first Pikmin. They are usually found inside cans, boots, or other cramped places. Once Olimar discovers the Yellow Pikmin, he notes that they like to carry around these volatile stones. When yellow pikmin are near them, use the C-stick or X button to make them pick them up. Now, pressing X will separate the yellow pikmin by whether or not they are carrying a bomb rock. If you throw one of these pikmin and immediately call it back, it will drop it's bomb and come back to you. After a second or two, the rock will explode and damage anything near it. Six bomb rocks are necessary to destroy rock walls that can't be destroyed by Pikmin alone. They can also be used to attack enemies. Just two of these bombs can kill a regular Bulborb. They are also needed to defeat the game's final boss, Emperor Bulblax.
 Pikmin 2
In Pikmin 2, they are no longer found laying around, and they cannot be picked up by yellow pikmin. Instead, they are only used by enemies such as the Careening Dirigibug and the Volatile Dweevil. They also tend to drop from the ceiling in some places while in Caves. If you are lacking white or yellow pikmin, you can lure these enemies over to poison and electric generators and power them down by letting the bomb rock explode near it. This tactic also works for taking out enemies, but is easiest when you are alone and don't have any pikmin following you.