A volunteer-run, free, naked, mixed sauna on the waterfront in the Sompasaari district. Open at all hours. Bring your own firewood (or use what is on the pile), feed the stove, swim in the harbour between sessions. No staff, no booking, no door — the etiquette is to introduce yourself and to leave it cleaner than you found it.
The location, on what used to be an industrial wasteland, has gradually been swallowed by new apartment blocks, but the sauna itself remains a defiantly unbranded experience in a city full of slicker alternatives. Tram 9 to Kalasatama or a 15-minute walk along the shore from Hakaniemi.