A ceiling under the ground
The puzzle
You walk into a building and its ceiling is below ground level.
The building is properly built, and it is not an optical illusion.
How can that be?
See the answer (spoiler)
Spoilers from here on. If you want to work it out yourself, leave this folded and move on.
Answer
The building is underground — an underground shopping street, say.
You go down a staircase to get in, so the whole thing, ceiling included, sits below the ground outside.
