Hardened Sand Wall

Underground Desert generates this behind most of its exposed stone. Breaking the natural version leaves you nothing to pick up, so the only way to hold the item is to make it yourself. That matters more than it sounds, because a copy you place counts as player-built and stops spawns, while the identical-looking natural wall does not. Walling off a desert base means replacing what is already there, not just standing in front of it.

The natural walls are not neutral terrain. Corruption, Crimson or Hallow reaching an Underground Desert repaints them into the matching hardened sand variant, and those converted walls then push the spread further on their own. If you are trenching to contain a biome, the walls count as part of the infection, and cutting only the blocks leaves it a route.

Drop the wall into Shimmer and you get the Treacherous Hardened Sand Wall, which looks identical but places the unsafe kind. That is the whole point. Builders use it for a desert interior that reads as natural cave and keeps spawning enemies, which is what you want in a farm and never what you want behind your beds.