Heightmap doesn't seem to be tall enough (Resolved)

I have imported a new heightmap into my level. Not quite sure how to phrase it, but there seems to be a grid slightly below the first person character, then the heightmap a couple hundred meters down. Once the player falls far enough to touch this grid, the level will stop.

Just trying to learn a bit more and figure out what this grid is, how to change it, and what it does.

Screenshots attached.

EDIT
I added a new level. I believe the issue was the killzone with the default template for the world.