Koivusaari metro station is a station located under water on the Länsimetro extension of the Helsinki Metro. Koivusaari is the only metro station in the world located under the sea. The station has 76 meter-long escalators and funicular-style elevators. It is the westernmost metro station in Helsinki. 

The free-form roof of the Koivusaari station compares with an upside down wooden vessel. The principal exterior cladding materials are acid-proof steel, patinated solid zinc sheet, glass and natural stone.

The floor and walls of the main entrance are red granite, which is the most common stone of the shoreline rocks of the area. The exposed roof beams are wood structures, and the roof is supported by a massive primary beam with a curve at one end.

The upper parts of the long side walls of the platform are composed of spotlighted wave-like “baleens”, which have both an atmospheric and an acoustic function.

The ceiling is asymmetric in every direction, and a work of art reflecting the sea is planned there. The flooring of the platform is on-site cast, polished concrete mosaic. Koivusaari is the only undersea metro station in the world and its escalator is the longest in Finland. 

According to helinco.fi