The National Museum in Helsinki is considered a symbol of the Finnish identity and is to be expanded. The proposed extension "Salah" (prefix for secret in Finnish) aims to generate an autonomous spatial identity resulting from the reading of the place, while interpreting the needs of a contemporary museum extension. The developed architecture intends to preserve the aura of the place, highlight the existing qualities and hide a rich and unexpected underworld behind a quiet Pavilion. Thus, Salah cultivates and celebrates the mystical secret of Helsinki, a huge hidden underground world.