So I beat the Golem and finally decided to buy the Clentaminator to clear the world of most of the more troublesome hallow and Crimson.
While clearing the chasms I found this background area that doesn't seem to budge to anything. The Clentaminator doesn't purify it, the Pwnhammer doesn't smash it, and the Pickaxe Axe doesn't seem to have any effect on it.
And while I did try searching for it on the web I didn't manage to find any info.
What is this thing?
![Cave with background[1]](https://i.stack.imgur.com/X4p4m.png)
After posting, I managed to find another one under the other Chasm, and they are destroyable by using bombs (that destroy tiles) and dynamite. So I guess they're just background tiles as Sable Dreamer claims.
Best Answer
Looks like just a natural cave wall (a new 1.3 texture, that may or may not only spawn near Crimson). Can you place torches on it? If so, it's a wall.
So, good news: Walls can't spread Corruption / Crimson, so even if this is due to proximity to crimson, its not "infected", just different looking.
The thing about walls, though, is that you can't destroy them from the middle -- you need to work your way from the outside in. Dynamite also destroys background walls, so I'd try lobbing a few sticks into this cavern and see if that blows any holes in the background. If it does, simply continue destruction from those holes, removing as much as you need to achieve peace of mind.