Minecraft wiki entry for Cave, associated text: Aquifers are flooded cave systems used to generate bodies of liquids in noise caves. Aquifers may create underground lakes.[3] Magma blocks may appear at the bottom of aquifers, creating bubble columns.[3] Because they are filled with water instead of air, aquifers often contain ores that generate with reduced or no air exposure like coal or diamonds. Glow squids are common in aquifers below Y=30, and tropical fish may spawn in any aquifers found in lush caves. Aquifers below Y=0 may generate filled with lava instead of water. The color of water in an aquifer depends on the biome containing it.