Minecraft wiki entry for Seed_(level_generation), associated text: If the seed contains characters other than numbers or is greater than or equal to 20 characters in length, the Java String.hashCode() function is used to generate a number seed. This restricts Minecraft to a subset of the possible worlds to 232 (or 4,294,967,296), due to the int datatype used. Number seeds or a default world seed must be used to access the full set of possible worlds (264, or 18,446,744,073,709,551,616).