Märket Lighthouse stands on a bare rock in the Sea of Åland, west of Eckerö. The skerry is famous for its border: Finland and Sweden share the island, and the line bends awkwardly around the lighthouse because it was built on the wrong side in the nineteenth century.
Trips usually run only in calm summer weather and take several hours, with the crossing as much a part of the experience as the lighthouse itself. Bring warm layers even on sunny days; there is little shelter once you are out on the skerry.