It’s a biome with grass, trees with no leaves, and stones arranged to look like headstones. If you dig under the headstones you can find gleam, growth, or sometimes just dirt. Some stones are arranged to form more elaborate tombs, which more often contain growth if you’re into that.
There are usually lots of monsters around. There tend to be many plants in these areas too, particularly trumpet roots and mushrooms (but not the shadow orb mushrooms).
Are they on T1 worlds, or just T3+?
I havent seen them on T3, but then again i havent searched the entirety of them all, so maybe. However, there are quite a few on the T4+ worlds.
Would placing a block down, instead of breaking blocks in the chunk, cause regeneration too? Placing a soil block down instead of mining one up for example
Yes if it is “out of place” the world will try to remove the “extra” block. Thereby initiating regen
Sweet, I have something to do with all that sultry soil hanging around!
Maybe auto breaking the block underneath the plant when harvested is a solution for the devs?
Good to see this is being addressed 
The no regeneration kind of makes sense in a limited way, but actually solving the underlying issue is much better!