Games Set in Caves are the new Games Not Set in Caves - I'm calling it now. Between Knytt Underground , Double Fine's The Cave , Cavenaut , and probably a million other gloomy splelunking games, the humble hole in the ground has been getting a lot of love of late. We can now add I GF nominee Kentucky Route Zero to the list. It's a beautiful adventure game about a "secret highway in the caves beneath Kentucky", and its first act has just been released for $7.
As to what kind of adventure game Kentucky is, you'll be pleased/unpleased to hear that it's "focused on characterization, atmosphere and storytelling rather than clever puzzles or challenges of skill", which suggests to me that it might be a bit like Superbrothers: Sword & Sworcery. However, being an episodic, five-act affair, we won't get to the bottom of Kentucky Route Zero until the remaining parts are released "over the next year or so". The optimistic among you can buy them all in one go, for $25, which will also bag you the accompanying soundtracks.
Here's a trailer. It's as cryptic as it is enigmatic, so it's very mysterious indeed.