30, is concert overture for orchestra written between 1939 and 1940 by the Finnish composer Uuno Klami, who had visited the eponymous island sea fortress.

[2] The piece was lost in Germany during the "chaos"[3] World War II, prompting Klami to rewrite it in 1944 based on his original sketches.

[4][2] Around this time, Klami renamed the overture Linna meren äärellä[5] (The Fortress on the Sea), but this name has never stuck.

The music critic Sulho Ranta argued that, although "the thematic material... [had] not greatly changed" between versions, "the revision [had] proved to be to the work's advantage".

[2][a] On 29 September 1950 in celebration of Klami's fiftieth birthday, Jussi Jalas and the Helsinki Philharmonic Orchestra performed the overture.