In chapter 19, "We Find Out the Truth, Sort Of," Percy, Annabeth, and Grover have just arrived in the Underworld. They see the huge masses of countless dead souls milling about listlessly in the Asphodel Fields. Beyond are the Fields of Punishment, in which the many souls of the wicked are subjected to various forms of eternal torture. On the other side of things, Percy notices a different sort of space. It is a gated community with fine-looking homes, beautiful gardens, and a large blue lake. This is Elysium, and it is the only happy place in all of the Underworld. The sounds of laughter and cooking meat float up from this place. In the lake are three islands, The Isles of the Blest, reserved for people who have been reborn three times and achieved Elysium each time. The souls of heroes reside in Elysium. Percy hopes to end up there someday.
Percy notices that Elysium is considerably smaller than the other sections of the Underworld. This causes him to pause and consider how few people end up in Elysium compared to the Asphodel Fields or the Fields of Punishment. This is a depressing realization that very few people choose to do good in their lives.