Minecraft wiki entry for Cave, associated text: During the terrain generation process, large symmetrical and cylindrical or spherical voids of various sizes can infrequently be created underground, which often merge with other cave systems. These unusual type of caves vary from roughly 1 to 27 blocks in height, and from roughly 13 to 30 blocks in diameter, though larger examples are quite rare. Although not particularly of interest, from a mining perspective, they may appear unnaturally regular in comparison to the surrounding terrain. While the smallest rarely contain many ores, the largest can be quite useful for mining as their sheer surface area rivals that of a large ravine.