Understen

Understen lighthouse
Understen
	The current lighthouse is the black and white tower in the centre
Location	Understen, Norrtälje Municipality, Sweden
Coordinates	60°16′30″N 18°55′12″E / 60.275113°N 18.919998°E
Tower
Constructed	1915
Construction	reinforced concrete
Automated	1966
Height	39 m (128 ft)
Shape	tapered 2-stage cylindrical tower with balcony and lantern
Markings	black (stripe), white, white (lantern)
Power source	kerosene, diesel generator, electricity
Operator	Swedish Maritime Administration
Light
Focal height	48 m (157 ft), 32 m (105 ft)
Lens	third order Fresnel lens
Range	23 nmi (43 km; 26 mi) (main light), 15 nmi (28 km; 17 mi) (auxiliary light)
Characteristic	Fl(4) W 15s, F WRG (1922–)
Sweden no.	SV-2201
Main light
First lit	1916
Focal height	48 m (157 ft)
Lens	third order Fresnel lens
Range	23 nmi (43 km; 26 mi)
Characteristic	Fl(4) W 15s
Auxiliary light
First lit	1916
Focal height	32 m (105 ft)
Range	15 nmi (28 km; 17 mi) (auxiliary light)
Characteristic	F WRG
Understen old lighthouse
	The old lighthouse is the unpainted tower.
Constructed	1847
Construction	natural stone
Height	16 m (52 ft)
Shape	cylindrical tower with balcony and lantern
Markings	grey
Power source	rapeseed oil, kerosene
Heritage	governmental listed building
First lit	11 November 1848
Deactivated	1916
Focal height	24.3 m (80 ft)
Lens	10 parabolic mirrors
Range	14 nmi (26 km; 16 mi)
Characteristic	Exting (1916–)

Understen is a Swedish island and lighthouse station located in South Kvarken, east of Öregrund at the northern tip of sea of Åland in Sweden.^[1]

History

The first day beacon on the island was built in the 18th century. The first lighthouse was built in 1848, and lit in November that year with a colza oil lamp. In 1880 a kerosene lamp was added but the lighthouse was considered too short (13 metres) and weak, and plans for the construction of a more powerful tower started. In 1916 the new much higher tower was completed and fitted with a powerful rotating Fresnel lens in 1922. The old tower was not demolished and survives to this day, but its light equipment was removed.

The keepers left their job in 1968, after the tower was automated in 1966. The island has been used as a station for the Swedish military due to its strategic location in the sea, and was staffed by military personnel in 1975–1996. On the island is also an observation tower built by the military. That tower is today fitted with remote controlled cameras observing the sea traffic in the area. The lighthouse remains in use and is still powered with electric cable and 1000 watt bulbs. It is owned by the Swedish Maritime Administration, and remote controlled from their headquarters in Norrköping. For many years, to visit the island permission had to be granted by the Swedish marine, but it is uncertain if this is valid nowadays (2010). The island can be observed on a distant view to the north from the ferries traveling between Grisslehamn and Ekerö on Åland.

SMA has confirmed that many Russian and Swedish mines dating from the first and second world war may still lie on the sea bottom east of the light station, making anchoring or diving dangerous in the area.^[2]