Zeus' Fist

Zeus' Fist is a pile of rocks in the forest of Camp Half-Blood. If you look at it from a certain angle it looks like a large fist. If you look at it from any other angle, it looks like a pile of deer droppings, so the campers are prohibited from calling it "the poop pile", as Zeus tends to be picky about things that bear his name and Chiron believes it disrespectful to call something a "pile of deer poop" after it was initially named after the king of the gods. It is used in Capture the Flag as a location for a flag, as it is clearly visible and hard to climb.

The Battle of the Labyrinth
In capture the flag Percy found out that Zeus' Fist also serves as an entrance into the Labyrinth. In The Battle of the Labyrinth, Luke used that entrance in the Battle of the Labyrinth to get his army into Camp Half Blood. There was a huge fight, but when Daedalus died, the Labyrinth was destroyed and the camp was saved.