In the book, The House of Hades, while Percy and Annabeth are stuck down in Tartertous, they almost describe it as a creature similar to the way Gaea is the Earth. They compare Tarterous to different living things and compare things in the prison to human features like teeth or a mouth. "We started their decent into the giant pit full of monsters. The closer and closer I got, the more and more the rocks looked like teeth and the giant cave like a mouth" (17% Riordan). This shows how either Tarterous has an evil god like Gaea, or they are really paranoid.