Mud Wall

Mud wall that was already there when you arrived is natural, and enemies spawn in front of it exactly as they would in open air. Wall you place yourself does not allow that. Both draw the same texture, so a Jungle base built inside an existing mud pocket keeps producing hornets in the living room until you break the old wall out and put your own back in its place.

Dark Blue Solution turns it into unsafe mushroom wall, which is one way to seed a glowing mushroom patch underground without hauling blocks in. Green Solution takes it back, though what you get depends on how deep you are: shallow enough and it returns as Jungle wall instead of mud.