Imagine a river so long that it touches Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, Romania, Moldova and Ukraine. That is more countries than most rivers in the world. Boats, barges and tourist ships chug along it every day, carrying goods and people between cities.
In Belgrade, the Danube meets a second big river called the Sava. The two rivers join right under an old hilltop fortress called Kalemegdan, where children fly kites and families walk along the walls at sunset. Standing up there, you can see both rivers stretching off in different directions.
Further east, the Danube squeezes through a narrow valley called the Iron Gates - a deep canyon where the river is hemmed in by tall limestone cliffs. The cliffs are home to eagles, and the rocks were carved with messages by people thousands of years ago.
The Danube is also a home to wildlife. Pelicans, herons and storks fish in its shallow side-channels. In winter, you can sometimes see ice floes drifting past Belgrade. In summer, swimmers jump in from floating rafts called 'splavs' that line the riverbanks.