The Romans were the inventors of sauna, which dates back to the 2nd century BCE. The floor of the baths rested on short, brick pillars (pilae) that allowed the hot air to circulate freely. Brick pipes lined the walls and the vaults all the way up to the ceiling for the same purpose. The southeast orientation of the baths also added to the heat as the afternoon sun came into these rooms through large (often double-glazed) windows.
In this hypocaust in Germany, excavators found the skeleton of a dead dog, which had apparently crawled in for warmth and was asphyxiated by the fumes.