Crazy Trench Glitch?

It's the second time I encounter on of those.

It's like a section of the world doesn't get rendered. It goes all the weay to the bottom.
The first time I went in and promptly fell to non-existence.

This time, I stayed on the edge.
And I tried building a bridge over it, which is where the 3rd picture is from.
There was land there before. It neat to see a cross-section of the world. I beleieve I spotted an old mine shaft. I may need to investigate.