Underground city world building generator game

What is the name of the solo game that can generate underground cities--it's from the 2000's, I think.

Some tunnels move outward, others move down, with the idea is that it grows over the course of "history" and you ultimately create a map that you can use as a resource for other games.

IIRC, there was a free version, and then they created a better or at least not-free version.

Something like this, but better detail and not ascii.

I was just reading about Cartography, and remembered that this existed too, and was a bit similar.

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