In the comic strip, the cities of the Eastern Seaboard of the United States had become so polluted that by the end of the twenty first century they were simply sealed over with concrete, and a new city was built on top. This city was to be named Mega City One, whilst the remains of the old city below became known as "the Undercity".
The undercity is inhabited by various mutants and criminals (or perps as they are known in the comic), and has featured in several Judge Dredd stories, the most significant of which have been The Day the Law Died, in which Dredd and a small band of rebel Judges (along with undercity denizen Fergee) lead the resistance against the insane Judge Cal, and Necropolis, in which Judges Dredd, Anderson, McGruder and a small band of Cadet Judges use the Big Smelly river (previously the Ohio River) to travel beneath the city in order to avoid and fight back against the Sisters of Death.