"Doc, what's past the city borders?"

"Nothing."

"Nothing that you know of? Or actually just nothing?"

"Always room for me to be wrong. But I mean nothing at all. Just a void, no map, no color, nothing. That's why there's no way to get out."

"But where does it go? When we hit the edge of the whole partition, is it just a big wall?"

"There's no edge to nothing. Nothing is nothing."

"I don't get it."

"Yeah, me neither."

Doc scratches the back of his head, failing to come up with a suitable explanation.

"When you imagine something inside your mind, can you touch the inside of your skull with that thought?"

"Uh..."

"What?"

"Oh, never mind, girl, ask something else!"

That's all you're getting regarding what's beyond the borders, then. Unsatisfying, but that's all there is. You're glad you didn't go any closer to it.