The narrow strait that once funded a kingdom and now links two nations

Countries & Cities · Published 30 September 2026 Swipe more like this →

In short: Separating Denmark from Sweden, the Øresund is a geologically young waterway that transformed the Baltic Sea into a salt-water basin. Beyond its ecological complexity, this narrow passage served as a critical economic engine for centuries, dictating the power dynamics of Northern Europe through its strategic tolls and military fortifications.

Formed roughly 8,000 to 8,500 years ago as sea levels rose, the Øresund serves as a vital conduit between the Baltic Sea and the North Sea. Its unique hydrology creates a sharp divide between the oceanic salt water of the north and the brackish environment of the Baltic. Because the southern portion of the strait is shallow, reaching only 5 to 6 metres in depth, it acts as a biological barrier. While the North Sea supports around 1,500 species, the central Baltic hosts only 52, with the Øresund acting as a transitional zone for roughly 600 species.

For centuries, control of this 118-kilometre strait was synonymous with regional dominance. In 1429, King Eric of Pomerania established the Sound Dues, a tax on ships passing through the waterway. These tolls became the primary revenue stream for the Danish Crown, granting monarchs significant financial autonomy from their aristocracy. The importance of this income was so profound that Sweden launched major infrastructure efforts, including the 1621 founding of Gothenburg and the 1810–1832 construction of the Göta Canal, specifically to bypass these levies.

The geopolitical landscape shifted in 1658 when the Treaty of Roskilde ceded the eastern shore to Sweden, ending Denmark's exclusive control over both sides of the narrowest point. By 1857, the Copenhagen Convention finally abolished the Sound Dues, opening the strait as an international waterway. Today, the region has evolved into a bi-national metropolitan hub. The inauguration of the Øresund Bridge in 2000 physically linked Copenhagen and Malmö, integrating a population of nearly 4 million people and replacing the historical friction of the strait with modern connectivity.

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