Taking a deep dive in Helsinki’s subterranean city

Under the Finnish capital lies a world of swimming pools, go-karting tracks, playgrounds – and even an art gallery The moon is an impostor. Swimming backstroke beneath its surface, I look up at craggy hollows in close relief.

White rock undulates in all directions, a forest of peaks rising next to a desert of craters. Every time I trick myself into thinking I’m somewhere celestial, the sound of children splashing brings me back to earth. This is no lunar landscape but Helsinki’s underground swimming hall, excavated from granite bedrock and officially opened in 1993. These…