The Rumbling Grotto is a Challenge Mode arena where you have to find the key on one sublevel and defeat a Pileated Snagret for it on the next. Proceed with caution.
Sublevel 1
The cave starts rumbling while Creeping Chrysanthemums, Cloaking Burrow-nits, Sheargrubs and Shearwigs come out of the ground. If you don't want to risk Pikmin deaths, you can reset the cave until the key lies right beside you, which happens quite often.
Sublevel 2
More rumbling with the Pileated Snagret. Use all sprays here so the Pikmin don't get harmed, grab the key, and jump out.
If you are doing a 2-player Challenge, the battle can be very quick and easy. Stand on opposite side of the Snagret and have both players toss Pikmin rapidly at its head. If you do it right, it should always go back into the ground instead of eating any Pikmin, and you save time and deaths.