Suppose that the shape of the earth was an almost perfect cube.
Sounds boring, right? But what if you start an expedition
at the north pole, which you may imagine to be in the center of the
top face. You travel easily and without obstruction on a perfectly
flat stretch of land and if you don't pay attention, you'll cross the edge.
That's scary. Actually, you will find it very difficult even to get to
the edge - and if you do manage to get there...