The experiment cave is a cave involved during the quest Creature of Fenkenstrain. It can be accessed by pushing the easternmost grave to the south-east of Fenkenstrain's Castle. During the quest, players have to kill a level 51 experiment to obtain the cavern key to the northern gate in the cave, which leads to the Mausoleum.
The cave is the only place in RuneScape that contains experiments. The experiments are level 25 and have an unusually high amount of health for their level. The cave is frequently used to train combat experience.