No such thing as a terrain generator... the reality is unless you actually have an extremely strong landscape engine there is no actual way to do "caves" or intrusions in terrain without very cheap and obvious trickery. To do caves, you need the ability to edit the terrain mesh directly, and most "normal" engines use simple height-maps. With no real ability to alter specific parts of a mesh in a 3d-editing style way (like extrude/bevel/etc...)
Terrain consists of more than just caves.
There is definitely such a thing as a terrain generator. Bethesda has been using it for years, as have many other developers. It randomly generates basic terrain, at which point the artists jump in and start tweaking things and making stuff look more natural. This is also the point where they start adding unique features like caves, rocks, trees or other structures.