Seven Bridges of Koenigsberg. This historical puzzle is based on the city of Koenigsberg in what was then Prussia (it is now Kaliningrad in Russia). Seven bridges over the Pregel River connected the two sides of the city and two islands in the river. The problem posed in the early 16th century was to find a single route that crossed each bridge only once. This problem was the subject of a 1735 paper by the Swiss mathematician Leonhard Euler (1707-1783), who showed that it was not possible to find such a route. This problem is an example of graph theory and modern topology, which both developed from Euler's pioneering work.