The Nothing is eating my world

What might be causing this? The last thing I edited was the terrain texture. I may have tweaked some other things like volumetric fog and the general directional light settings, but I’m not sure. It only happens after about 30 seconds of PIE.

Possibly related and totally bizarre, I was getting mentally prepared to try to figure out how make an unlit object cast shadows through various tricks, then when I got up this morning my unlit character was casting shadows:

I eventually worked out that the scene is quite dark, but there’s a lot of volumetric lighting being cast around. The shadow is simply the place where the ground remains unlit. UE continues to impress me with its accidental advances.