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A caldarium was a room with a hot plunge bath, used in a Roman bath complex.

This was a very hot and steamy room heated by an underfloor heating system. This was the hottest room in the regular sequence of bathing rooms; after the caldarium, bathers would progress back through the tepidarium to the frigidarium.

In the caldarium, there would be a bath of hot water sunk into the floor and there was sometimes even a laconicum, a hot, dry area for inducing sweating.