Metropolis is vastly overpopulated and has 1 trillion people so sex was banned in this city. It is also the Mars Bars capital of Solona and is on Kerwan. It was not nuked but instead tons of people were made Unsquishable by Dr. Malarious with the Biobliterator (a crap copy of the Obliterator) so all except immigrants and illegal retarded immigrants in the city are squishables that smoke crack cociane or otherwise eat Mars Bars.

Ratchet & Clank wiki
For those without comedic tastes, the so-called experts at the Ratchet & Clank wiki have an article about Metropolis.