Rock lobsters are found in the Waterbirth Island Dungeon's fifth sub-level, in room 16. They are similar to Giant Rock Crabs in the fact that they are initially disguised as large rocks, but they will attack the player as they approach. They have very high defence (though they are weak to magic attacks) and can hit up to 150 life points, but rarely hit over 110 life points. Their attack speed is twice as fast as any "Fastest" speed weapon (equivalent to dual-wielding such weapons), making them incredibly dangerous. Rock lobsters are commonly killed by wealthy players because of their common crimson charm drops. However with the use of charged battle robes, a reward from the Fist of Guthix minigame, the price is significantly decreased. Using the right technique, such as ice bursting or ice barraging, killing rock lobsters is one of (if not) the fastest way(s) to obtain charms in RuneScape.
The rock lobster's combat level was 127 upon being introduced into RuneScape 2. It was later changed to 156 in a hidden update. Their combat levels changed again to 142 upon the release of the Evolution of Combat on November 202012.
If you manage to get the Rock Lobster within a distance while you're running to another place, sometimes, the intact version (the disguised as a rock) will be seen moving.