Märket ("The Mark", ) is a small uninhabited skerry in the Baltic Sea shared by Sweden and Finland (in the area of the autonomous region Åland), with a lighthouse as its salient humanmade feature. Märket has been divided between the two countries since the Treaty of Fredrikshamn of 1809 defined the border between Sweden and the Russian Empire as going through the middle of the island. The Finnish side of the island is part of the Municipality of Hammarland and is the westernmost land point of Finland. The Swedish part of the island is itself divided by two counties of Sweden: Uppsala County (Östhammar Municipality) and Stockholm County (Norrtälje Municipality). Geography and history The Understen–Märket Passage links the Bothnian Sea to the Baltic proper. The skerry is roughly long by wide, and its area is about . It is the smallest sea island shared by two countries. It used to have a straight border until the Grand Duchy of Finland built the lighthouse on the Swedish side and then, they had to change the border. The name Märket ('the Mark') probably comes from its usefulness as a navigation mark before there were lighthouses. The route between Sweden and Åland has a passage about long over open sea. Before the lighthouse was erected, the island and its shallows were dangerous navigational hazards, which seafarers tried desperately to avoid. In 1873, as many as 23 ships were grounded on the Swedish coast and its archipelago trying to avoid Märket, and eight of them were shipwrecked. Märket is detached from the main Åland archipelago, with the closest island more than away, and the closest harbor, Berghamn, away in Eckerö. There is no deep harbor; the island can only be reached with boats. There are small, barely surfacing rocks northwest of Märket, called Märketshällor ("stones of Märket"), which are too small to sustain vegetation. The island consists of mostly smooth diabase rock, with a maximum natural elevation of . Most of the area is regularly washed over with seawater in storms, and scoured by drift ice in winter. Plant life, which is limited to low-growing grasses and herbs, persists only in some protected spots. Twenty-three plant species have been identified altogether. The halophilic grass Puccinellia capillaris and the herbaceous Sagina nodosa (knotted pearlwort) are found scattered throughout the island. Among rarer species, Spergularia marina (salt sandspurry) grows on Märket. Salix caprea (goat willow) grows