Task about the plane

I do not know how well this task is known, but I think those who suddenly did not know about it will like it.

“The plane flew 100 km strictly to the south, then 100 km to the west and 100 km to the north, after which it found itself at the starting point. Where did he fly from? ”

The decision is not one!
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Land is considered as an ideal sphere, neglect the height of the aircraft.