The House of Hades by Rick Riordan is the fourth book in fast-paced fantasy-adventure series The Heroes of Olympus. The books follow seven teenage demigods (someone of human and godly pedigree) of both Greek and Roman decent as they attempt to thwart Mother Earth, Gaea, and her grandiose plans of raising her children, the Giants (enormous humanoid beasts), and attacking Mount Olympus in Greece to completely usurp control of the world from the Olympians and throw the world into turmoil. Towards the end of book, two of the main characters, Percy and Annabeth, along with Bob the Titan (formerly known as Iapetus before he was brainwashed by the memory-stealing waters of the River Lethe in the Underworld) arrive at the Doors of Death at the heart of Tartarus, the nearly bottomless pit where monsters reform after they have been killed in the mortal world. The Doors of the Death, normally controlled by the god of Death, Thanatos, are being controlled by Gaea so she can allow countless monsters to re-enter the mortal world to help her capture Olympus. Percy and Annabeth need to get out of Tartarus so they can stop this, but it is not as easy as it seems. When Bob the Titan tells Percy and Annabeth that the UP button on the Doors of Death (they are designed like elevators) needs to be held down for twelve minutes in order for the passengers to reach the mortal world, Percy has an Aha Moment. Because the Doors of Death are where all monsters re-enter the mortal world, there is virtually an army of monsters with an unrelenting hate of demigods waiting there. The Doors of Death themselves are designed like an elevator and are guarded by two Titans, and to successfully transport from Tartarus to the mortal world, the UP button of the Doors needs to be pushed for twelve minutes. This changes things for Percy because when he realized how unlikely it is for even one person to get through the Doors, it means that he is probably ready and willing to sacrifice himself and stay behind so that Annabeth can escape relatively unscathed.