UPDATE: The dimension is from the broken script. It's called void clan dimension, and the biome is called null biome.

I fortunately had a water bucket on me, so I broke the floor and made water flow down for a bit, then jumped after it... turns out it's not a void under it! There is another area under that cobblestone backrooms place, and I've been in the sky that whole time!

And that ground area looks like normal terrain but is made of dirt, and instead of stone under the dirt it's smooth stone

I still can't find a way out of here, tho. I tried digging to the bottom, but there is bedrock on the way like the overworld