One of the earliest problems of graph theory, and the problem for the solution of which the beginnings of graph theory were invented, was the Konigsberg Bridge Problem. The problem was as follows: The city of Konigsberg rests upon a river. Here is the picture of Konigsberg.

There are many beautiful bridges from the islands in the river to each other and to the river's banks, as emphasized below

The Konigsburg Bridge problem was first solved by the great 18th century mathematician Leonhard Euler, and the generalization of his solution to all graphs came to be one of the many, many facts proven by that same person, and is known as Euler's Theorem.