Minecraft wiki entry for Monolith, associated text: The monoliths would cause the terrain to abruptly generate up to the height limit, with natural grass block and ore generation. They could theoretically generate infinitely upward, being stopped only by the height limit, which at this point in the game's development, was 128 blocks. The area below the monoliths was completely hollow, except for water generating at sea level and a layer of bedrock at the bottom, making the normal terrain seem like inverse monoliths. It is possible to find small crevices in large monoliths, where normal terrain was generated.