Minecraft wiki entry for History, associated text: Version: 1 byte (may be 1 or 8)
Storage count: 1 byte (omitted if version is set to 1)
Zero or more concatenated Block Storage records, as specified by the count (or 1 if version is set to 1).
In the majority of cases, there is only one storage record. A second record may be present to indicate block water-logging. Blocks that are not water-logged are given the "air" block.