This evening the TV show “Numbers” mentioned the famous “Seven Bridges of Königsberg” math problem. Königsberg in East Prussia
included two large islands which were connected to each other and the mainland by seven bridges. The question is whether it is possible to walk with a route that crosses each bridge exactly once, and return to the starting point. In 1736, Leonhard Euler proved that it was not possible.
The islands are still there, in Kaliningrad, Russia, though some of the bridges are gone.