Minecraft wiki entry for Bubble_Column, associated text: Players and other air-breathing mobs can enter a bubble column to replenish their air supply, at the same speed that they would when they leave the water entirely. Water-breathing mobs avoid bubble columns because they suffocate if trapped in one. A bubble column is destroyed when moved by a piston. Bubble columns propagate only through water source blocks, stopping at any flowing water or waterlogged block. The height of a bubble column is limited only by the water's surface or by any obstructing blocks underwater.