I doubt it since it would probably require a complete rework - but why not add "fake" tunnels?

For example a craftable "grass plate" that will be recognized as ground as long as its connected to the ground at some point (aka looks like ground but actually is recognized by the game as a player building)?

edit: i actually like that caves etc are instances since this opens up a lot of great opportunities for modders to add random caves/dungeons etc. as a new instance.