Chicago is a city in Northeastern Illinois. It is the largest city in the state, and the third-largest in the United States, with 2.7 million residents. It's among the top five most economically powerful cities in the world, according to Forbes.
Leo Valdez, Piper McLean, and Jason Grace visit the city during their Quest to Free Hera. They go to a mall owned by Medea, a servant of Gaea, underneath the crown fountain. Piper defeated Medea by combining potions that caused the mall to explode. Jason, Piper, and Leo escape on Festus.