Understen, Lighthouse complex on Understen islet, Sweden

Understen is a lighthouse complex on an islet in the Baltic Sea, rising above the northern waters of the Åland Sea. The distinctive black and white tower features a cylindrical structure with a balcony and lantern room at the top.

The first lighthouse was completed in 1848 and lit in November that year with a colza oil lamp; a kerosene lamp was added in 1880. Because the 13-metre stone tower was too short and weak for the South Kvarken, a new concrete tower designed by Folke Lundberg was begun in 1915 and completed in 1916, reaching 39 metres. It received its present Fresnel lens in 1922 and was automated in 1966.

The island was staffed by Swedish military personnel from 1975 to 1996 because of its position where the Åland Sea meets the South Kvarken. A 28-metre observation tower built on the island was used to watch ships for the navy into the 1990s. A later sea-surveillance tower, originally built by the military in the 1970s, now carries cameras with advanced optics for remote monitoring of traffic in the Åland Sea.

The island can be seen from the ferries between Grisslehamn and Ekerö on Åland. Permission from the Swedish marine was long required to land, and it is uncertain whether this still applies. Mines from the two world wars may remain on the seabed east of the light station, making anchoring and diving dangerous there.

The system uses two separate lights with different ranges and color patterns to guide ships safely. The main light reaches a significant distance across the water and is supported by color-coded signals from an auxiliary beacon.