There is a section of terrain that is not visible but seems to exist
since if I try to move into the hole i will "bounce" back and sometimes
take damage like I am suffocating.
It is a 16 by 16 perfect square and extends (from what I can see) all the
way down to bedrock. Looks like water from my viewpoint.
In the picture you can see torches about halfway down; I'm pretty sure that
is a shaft that I found and the torches were placed by me;
but have not been down there since noticing the empty space.
I can't be positive but I'm pretty sure the hole was not there
at world creation. Especially if those torches were placed by me. I think i would have noticed the hole.