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...)
This is also why in Skyrim you see absolutely no "great" open cave systems with over-arching architecture or land mass.
Theres some caves and solitude but these are actual models intersecting with the real terrain, and not part of the terrain...
Or basically said, to have real terrain you need to literally make the terrain in a 3D-App and not by height-map.. that means exactly 1 person can make the terrain, or you spend 2 months to make sure it "connects" properly and all the sectors align to the neighbors perfectly. (because obviously you need to make terrain partitions, as 1 huge world is far too large to edit directly)