Nolybab

Nolybab is a large city built underground in a giant crater. Oddworld is a planet of multiple layers and many inhabitants reside underground, some very deep within the planet itself. The planet is almost "hollow" being built up of said layers, different ones varying in gravity, temperature, humidity, and such.

Alf describes Nolybab as "a city full of glukks", indicating that either they are the only or majority habitants of this city. It's also revealed that Sam, the Mudokon queen/mother of all Mudokons, is currently residing within Nolybab in the Good Ending of Soulstorm.

There are many other cities in craters and, looking over the landscape, they can be seen just poking out of these craters/holes/volcanic crusts.

Trivia

 * Nolybab is "Babylon" backwards, very likely inspired by the ancient city of Babylon