In chapter 5 of The Lightning Thief, Percy learns many surprising truths about the Olympians. Mr. Brunner, who it turns out is actually Chiron, tells Percy that the Olympian gods are alive and in America. Naturally, Percy is skeptical. Despite what he has been through already, Percy is not sure if he should believe Chiron. He thinks that the Greek gods are just stories and myths, "what people believed before there was science." This response does not impress Chiron, who goes on to lay out evidence that not only do the gods exist, but they are in America.
Chiron explains that the gods move around quite regularly. They settle in places where the "fire" of Western civilization is the strongest. It started in Greece. For a time the gods lived in Rome and then elsewhere in Europe. At the moment, they are in America.
As evidence of this, Chiron tells Percy to examine art and architecture. The gods can be seen in paintings, statutes, and in the architecture of significant American buildings. Chiron mentions the statue of Prometheus in Rockefeller Center and the Greek facades of public buildings in Washington, DC and throughout the country. In fact, the symbol of the United States, the eagle, is also a symbol of Zeus. Because the United States is "now the heart of the flame ... the great power of the West," the gods have taken up residence in America.