The Eternal City is a large underground city that lies beneath London, more specifically, the Thames River. It is huge, expanding a huge cavern underground, with many features. However, it is abandoned and filled with plague from the Plague Snails that thrive there. Despite being abandoned, it is guarded by the Styx Division patrols with their Stalker dogs. Will Burrows and his brother Cal enter it during book 1, and it appears in some of the later books.