This is Universe 42-C
This universe is a realm where the laws of physics are a mere suggestion. The fundamental forces are in a state of constant flux, causing the fabric of space-time to wobble like a bowl of jelly.
Consequently, the inhabitants of this universe have developed a unique relationship with the concept of "up". It's more of a suggestion, really.
They've built their cities with inverted ceilings and have mastered the art of "downwards-only" transportation. The phrase "up" is now a quaint relic of a bygone era.