Limestone Caves are a mini-biome added by the Polarities Mod. Similarly to the Granite Caves and Marble Caves of vanilla Terraria, it takes the form of a small enclosure of limestone blocks, roughly circular in shape and flooded with a thin layer of water or lava at the bottom depending on its height. In addition to the enemies that spawn within, it also generates stalactite projectiles that fall down constantly around the player and shatter on impact with the ground, dealing heavy damage that makes them a sizable threat, especially to early-game characters without much defense.
From Stalag Beetles:
From terrain and Limeshells: