Incomplete U-shaped cave in First World War base I:20 in [Krepost Sveaborg](https://en.wikipedia.org/wiki/Krepost_Sveaborg) land front defense line in Mustavuori, Vuosaari, Helsinki, Finland. The base was built in 1915 - 1918.
More info:
* [Location in Google Maps](https://goo.gl/maps/CtHVrWvWGiC1b1vT7)
* [Wikipedia: Krepost Sveaborg](https://en.wikipedia.org/wiki/Krepost_Sveaborg)
* [John Lagerstedt , Markku Saari: Krepost Sveaborg](http://www.novision.fi/viapori/eavaus.htm)
* [Finnish Heritage Agency site info](https://www.kyppi.fi/to.aspx?id=112.1000007641) (only in finnish)