Sure, the Underdeep is real. It's really big! There are roads down there, and traps, and all sorts of things that any monster with half a soul would fear. But there's also treasures left over by the titans; it's their land, their home, their hell.- Unknown
The Underdeep is a network of ancient underground channels, pits, access tunnels, and caverns, that lay beneath the "Topside" of MONSTRU. It is fairly massive, stretching from one side of the continent to the other, and it's depths are completely unknown.
It is a legendary place, one that most residents don't believe in, though most are aware of it in the childhood stories of "Darkbad".
There are at least four known "gates" to the Underdeep, each protected massive bars to prevent access. This was done some time ago by the AUTHORITY, to stop random creatures from wandering in.
Aside from the four gates, there are many other lesser-known entrances, including the massive fissure that divides MONSTRU into two halves. While one can't simply drive anywhere into the fissure, there are a few access points known only to locals.
History
The Underdeep was created and maintained by the Gorgoth, an ancient race of monster-titans. They carved up the interiors of MONSTRU to make a seemingly endless city. No one knows why the Gorgoth left, or where they went, but they left behind a massive network of subterranean bridges, chambers, porticoes and traps. Indeed, they even left some of their treasures behind.