The bastu. In the 1500s, a Swedish writer described men and women bathing in them together. Then syphilis spread, and doctors blamed the baths. Firewood ran short, and economists blamed the baths too. So the bathhouses were closed. By the early 1800s, bathing in a bastu was completely forgotten in Sweden.

Almost. From the 1580s, farmers from the east had moved into Sweden's deep forests, seven years without taxes. They burned the forest to farm it. And in their villages, the bath was never forgotten. They were Finns.

So Sweden, if you ever wonder where the bastu went: we were just keeping it warm for you.

Did your grandparents have a bastu?

Sources: Jane Fredlund, SO-rummet, "Personlig hygien förr": bathhouses banned in Sweden in 1725 (syphilis; a shortage of fuel and water); by the early 1800s sauna bathing forgotten "except in the distinctly Finnish settlements"; Olaus Magnus in the 1500s on men and women bathing together. Hans Albin Larsson (ed.), Boken om Sveriges historia (1999), p. 170, via Swedish Wikipedia, Badhus. Swedish Finn Historical Society (economists and firewood, doctors and venereal disease). Wikipedia, Forest Finns (settlement from the 1580s, seven years tax-free, slash-and-burn).
Sounds: Freesound (CC0): LiezelDippenaar, LampEight, vumseplutten1709, kyles, Vrymaa, justjare; the löyly: TRP, tonalt. Forest: Yle Arkisto, CC BY 4.0.