A sort of metal tube that goes all around the world (well, around it in a 'circle').
There are toilets and windows, food, showers, beds every 'certain distance'.
It feels as though you are just walking straight ahead, but in the end, when you get back to where you started, you will have actually walked around the world!
You wouldn't have to climb any hills/mountains or anything - just go straight ahead.

How long would this take to walk?

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