Render issue; sunk into black

Ugh, the laggy landscape rendering appears to have finally gone critical.  Was traveling by boat in the ocean and sailed into un-rendered territory.  Landscape went dark with just a hint of outline.  Appeared to be sinking.  After a few seconds the landscape went entirely black.  Placed a torch and found myself in a cave at y=23, deep under the ocean.

I do hope you can nip this rendering issue in the butt soon, as it really is putting a damper on an otherwise fine release.