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 (though this, differently from the piscina natatoria, was usually covered). The water could be also kept cold by using snow.
The frigidarium was usually located on the northern side of the baths. The largest examples of frigidarium were both in Rome: that of the Baths of Caracalla, located soon after the entrance, measures 58 x 24 m, and that of the Baths of Diocletian, covered by a cross vault. Some, like one in Pompeii, had a circular plan.