The city of Königsberg in Prussia (now Kaliningrad, Russia) was set on both sides of the Pregel River, and included two large islands which were connected to each other, or to the two mainland portions of the city, by seven bridges.
The Seven Bridges
The Seven Bridges of Königsberg is a historically notable problem in mathematics.
The problem was to devise a walk through the city that would cross each of those bridges once and only once.
Its negative resolution by Leonhard Euler in 1736 laid the foundations of graph theory and prefigured the idea of topology.