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 a hypocaust

Hypocaust

A hypocaust was an ancient Roman system of underfloor heating, used to heat houses with hot air. The word derives from the Ancient Greek hypo meaning "under" and caust-, meaning "burnt"...

, an underfloor heating

Underfloor heating

Underfloor heating and cooling is a form of central heating and cooling which achieves indoor climate control for thermal comfort using conduction, radiation and convection...

 system. This was the hottest room in the regular sequence of bathing rooms; after the caldarium, bathers would progress back through the tepidarium

Tepidarium

The tepidarium was the warm bathroom of the Roman baths heated by a hypocaust or underfloor heating system.The specialty of a tepidarium is the pleasant feeling of constant radiant heat which directly affects the human body from the walls and floor.There is an interesting example at Pompeii; this...

 to the frigidarium

Frigidarium

A frigidarium is a large cold pool of Roman baths. It would be entered after the Caldarium and the Tepidarium, which were used to open the pores of the skin. The cold water would close the pores. There would be a small pool of cold water or sometimes a large Swimming pool...

.

In the caldarium, there would be a bath (alveus, piscina calida or solium) of hot water sunk into the floor and there was sometimes even a laconicum—a hot, dry area for inducing sweating.

The bath's patrons would use olive oil

Olive oil

Olive oil is an oil obtained from the olive , a traditional tree crop of the Mediterranean Basin. It is commonly used in cooking, cosmetics, pharmaceuticals, and soaps and as a fuel for traditional oil lamps...

 to cleanse themselves by applying it to their bodies and using a strigil

Strigil

A strigil was a small, curved, metal tool used in ancient Greece and Rome to scrape dirt and sweat from the body before effective soaps became available. First perfumed oil was applied to the skin, and then it would be scraped off, along with the dirt. For wealthier people, this process was often...

 to remove the excess.

In modern gym

Gym

The word γυμνάσιον was used in Ancient Greece, that mean a locality for both physical and intellectual education of young men...

s and spas

Destination spa

A destination spa is a short term residential/lodging facility with the primary purpose of providing individual services for spa-goers to develop healthy habits. Historically many such spas were developed at the location of natural hot springs or sources of mineral waters...

, a caldarium is a room with a hot floor.