Tentacle pool

A tentacle pool is a pool of slimey water that can be built in an obulette. It requires level 71 construction, along with 100,000 Gold pieces and 20 buckets of water. When it is built, there will be a slimey pool of water in the cage of your obulette. When challenge mode is on in the dungeon, players who fall in the dungeon will be attacked by a tentacle-like creature, and it can inflict from 0-4 damage. It cannot be attacked back. If challenge mode is NOT on, however, the player will just 'swim' in the pool.

Trivia
The tentacle pool seems similar to the 'kracken' seen in Pirates of the carribian:Dead man's chest, as the 'kracken' is very rarely seen in full, and if the tentacle is examined, the character says it was glad it can only see that part of the creature.