In my level, the terrain mesh is almost the whole GG terrain, with exception of the border than will be water, later on.
The cave runs almost the entire perimeter underground.
And the idea is to build an underworld there, which will serve as a refuge and place to trade since everything is contaminated outside.
Really cave can be fake, even the hole on terrain can be fake too as you can see in my pic above.
The point is that if the terrain is not built with the idea of allowing you to dig into it, from the beginning, then it will be more difficult to add this as a feature, in the future.
Laptop: Lenovo - Intel(R) Celeron(R) CPU 1005M @ 1.90GHz
OS: Windows 10 (64) - Ram: 4 gb - Hd: 283 gb - Video card: Intel(R) HD Graphics
cpu mark: 10396.6
2d graphics mark: 947.9
3d graphics mark: 8310.9
memory mark 2584.8
Disk mark: 1146.3
Passmark rating: 3662.4