Southeastern Michigan, United States of America
Detroit is the largest city in the U.S. state of Michigan. It is currently the 18th largest city in the United States. The Hyperborean cyclops Ma Gasket and her two sons have a factory there; in which Leo, Piper, and Jason ran into in The Lost Hero.
Festus crash-lands and Leo, Piper and Jason fall into a closed car plant. Piper recovers from a hurt ankle while Leo is looking for Festus. Jason then goes looking for Leo, who he thinks is gone missing, leaving Piper alone. Jason is then knocked unconscious by one of the Hyperborean Cyclopes. The Cyclopes eventually capture Piper as well. Leo is able to save the two using his fire powers after being contacted by a sleeping woman in the earth (Gaea), who is the real reason for his mother's death. Festus's wires are replaced and the trio gets underway just as the Cyclopes reform.