The Labyrinth is a secret passageway that Hephaestus built when working on the prison of Tartarus. Because he built it from the outside in, he needed a secret passage to get himself out when the work was finished. This was to make the prison most impregnable. It is also the only way that a mortal can enter Tartarus. The floor of the Labyrinth continually moves, and at the end is the Minotaur, the final gatekeeper in this way to Tartarus.