User blog comment:JakeTehCrabsquis/Theories./@comment-30868164-20170306144418

To explain the void, I would say that due to volcanic activity and tectonic plates shifting, the playable area of land was pushed upwards towards the surface. This would explain why there is an active/inactive lava zone at the bottom and so many Basalt Chunks, which when scanned say they are from past volcanic activity.