Middelgrundsfortet is a sea fortress located in the Øresund between Copenhagen and Malmö. It was constructed 1890–94 as a part of Copenhagen's sea-fortifications, partly from material excavated from Frihavnen. It was the largest sea fortress in the World, and is still the largest man-made island without abutment, with an area, including wave breakers, of approximately 70 000 m²; the buildings total approximately 15 000 m². A HAWK battery was placed on the island. Until 1984 the fortress was military area; in 2002 it was sold to a private investor.
The fortress is constructed at a seabed 7 meter below the water surface, at the northern meeting point of the straits Kongedybet and Hollænderdybet.
Ferries connect Middelgrundsfortet with Langelinie, Copenhagen.
