Uhh, I dug into the underworld or something.
I started getting weird terrain geometry after reaching level 29 and it all went downhill (huehue) from there. I dug through the bedrock (black layer) to reach THE LIGHT (white layer), all the while the rest of the terrain was having a really bad day.
By the way, mushrooms are absolutely busted. I played for 3-4 hours straight that run and I wanted to lose so I could stop, but the shrooms wouldn't let me.