The Øresund Link connecting Copenhagen, Denmark, and Malmö, Sweden, looks like a sci-fi illusion from the air. Drivers start on an eight-kilometer cable-stayed bridge, arrive at an artificial island named Peberholm, and then dive directly into the four-kilometer Drogden Tunnel beneath the sea floor. Why not build a bridge the entire way and save billions in construction costs?
The answer comes down to a conflict between airplanes and cargo ships. Copenhagen Airport at Kastrup sits right on the Danish coastline, directly adjacent to the strait. If engineers had constructed bridge towers tall enough on the Danish side to let giant ocean vessels pass underneath, those massive steel pylons would have intruded directly into the flight paths of landing and taking-off aircraft, creating an unacceptable collision hazard.
On the flip side, building a low-slung bridge would have entirely blocked maritime traffic through the heavily navigated Drogden Channel. The tunnel was the only compromise that satisfied both air safety regulators and international shipping lanes. To seamlessly transition thousands of vehicles and trains from high-altitude bridge decks down into underwater tunnels, engineers dredged nearly two million cubic meters of rock and sand from the seabed to construct Peberholm.
Cheekily named 'Pepper Islet' to complement the neighboring natural island Saltholm ('Salt Islet'), this artificial landmass unexpectedly became a biological paradise. Because human access is strictly prohibited outside of emergency maintenance on the road, Peberholm was left to colonize naturally. Today, it thrives as an open-air ecological laboratory home to rare breeding birds, green-toads, and dozens of unique plant species that migrated entirely by wind and sea.