Suppose that the shape of the earth was an almost perfect cube.
Sounds boring, right? But imagine it, and that you start an expedition
at the north pole, which you may take 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 might cross 
the edge and perhaps tumble into the abyss. Pretty scary. Actually, 
you will find it very difficult even to get to the edge - and if you do 
manage to get there...