Experiment Cave

From the RuneScape Wiki, the wiki for all things RuneScape

The Experiment cave is a popular combat training area. The area is exclusive to those who have started Creature of Fenkenstrain, accessed by entering the tombstone northeast of Canifis. The cave is filled with experiments which accounts for its popularity, as the experiments have very low defence and high hitpoints, allowing players of all levels to acquire large amounts of combat experience in a relatively short space of time. Players who have completed the quest cannot train on the level-51 horse-like experiments, you will get a message: You dont have the heart to kill this poor creature again. Note: This will occur as well if you've got a cavern key in your inventory. Drop this key, and you will be able to kill the monster again, if that is the case.

The cave is usually absolutely flooded with players on popular worlds and unpopular worlds. Players often try to secure agreements with other active players on who gets to kill which monster to avoid Kill stealing.