Zeus' Fist (Labyrinth Entrance)

In The Battle of the Labyrinth Percy and Annabeth fell into Zeus' Fist (A bunch of rocks) and found one entrance into the Labyrinth. They only stayed for a couple of minutes, but since time runs slower in the Labyrinth, they were gone for hours in the above world. (And to Kronos' army) It was also used as an easy way into Camp Half Blood.