Underground grass + no Stone under stone [SOLVED]

[Geo]

OK so I'm playing a mountains map and I decide to dig down into some stone like so:


However underneath I find MUD! Not only that but the floor is grass! (Also the wall mud blocks are labelled as glass under the cursor.) Also I can till the 'undergrass' and grow trees underground

[Aquifel]

That may be a bug, but that actually looks like a very pleasant place to lounge around.

[Espero]

That was going on before 46. I had tree farms underground and would till dirt then untill it to turn it into grass for looks.

[supermalparit]

That's correct, the mountains map generation needs to be checked

[supermalparit]

Oh, we solved the map generation for mountains.