The Rhine River springs from the Alps in Switzerland, flowing through Germany, before it meets the North Sea in the Netherlands. After Julius Caesar's conquest of Gaul in 51 BC, the river would serve as Rome's western border in Northern Europe for most of the later Empire's existence. The Rhine is an important shipping route and its waters have been used to transport people and goods since antiquity. The river is 768 miles long.
Caesar twice built a bridge across the river's expanse to punish the Germanic tribes raiding Gaul, and to demonstrate Roman power. Here is an artist's rendering: