Bomb-Rocks are explosive rocks in the Pikmin games. In the first game, they can be carried by Yellow Pikmin, where their main purpose is to destroy rock walls. They can also be used to defeat enemies, and are particularly useful for doing so with bosses. In the second game, however, they are much larger and so cannot be carried (with the exception of when you are in the the Hostile Territory stage in 2-Player Mode: you can pick them up with your pilots), only detonated. In certain dungeons, they can actually be eaten by any nearby Emperor Bulblaxes in the area. They are also produced by Careening Dirigibugs. Volatile Dweevils carry only bomb-rocks, presumable for protection, even though they themselves are killed in the blast. Never throw Purple Pikmin on these, as that can cause near-instant detonation. Bomb-rocks can even be petrified, so do that if you are in a pinch, but keep in mind that they will not release nectar or spray like many beasts do if killed in that state.
In Pikmin 2, you can pick up a bomb-rock by nudging one with a Captain, even if its timer has already been activated. This can be thrown by pressing A or just dropped with B, which still activates it. The latter method of setting it off is handy for a more accurate attack on an enemy. Again, this only possible in 2-Player mode. Although it is possible to nudge the bomb rocks around in story mode, but the captains won't pick them up.