The Rhine is one of the major European rivers, beginning in the Swiss Alps and flowing 1,230 kilometers through Switzerland, Liechtenstein, Austria, Germany, France, and the Netherlands before emptying into the North Sea. It is arguably the most important commercial waterway in the world relative to its size, serving as a primary transport route for the industrial heartland of Europe, including the Ruhr region in Germany. The river has a storied history, having served as the boundary of the Roman Empire. Its banks are lined with hundreds of medieval castles and historic vineyards, particularly in the Middle Rhine Valley, which is a UNESCO World Heritage site. In 1986, a major chemical spill caused an ecological disaster in the river, but intensive international cooperation has since made the Rhine one of the cleanest industrial rivers in the world, with salmon returning to spawn in its upper reaches.