16x16 area Dark area opens to bottom of world believe it goes to ceiling. building next to it causes same black color on new blocks

*What I expected to happen was...:Flat cobblestone area
Bottom is blue but can see into caverns in wall
Render was at 16 chunks changed to 10, no help. reboot= no change
What actually happened was...:
Describe what happened here

Steps to Reproduce:
1. unknown, Entering game after several time, this time this part did not render this time
2. ...It is now part of the world.
3. ...Part 2. Found that part of a mountain in the same vicinity i removed blocks but on reboot they are back