[I must have heard this problem ages ago, but as I remembered it, one was always satisfied after finding just one solution. It was a math professor at the Kaiserslautern Technical University who brought asking for *all* solutions to my attention.]

Initially, you’re somewhere on the surface of the Earth. You travel one
kilometer South, then one kilometer East, then one kilometer North. You
then find yourself back at the initial position. Describe *all*
initial locations from which this is possible.

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