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The impluvium is the sunken part of the atrium in a Greek or Roman house (domus). Designed to carry away the rainwater coming through the compluvium of the roof, it is usually made of marble and placed about 30 cm below the floor of the atrium.
Inspection (without excavation) of impluvia in Paestum, Pompeii and Rome by an American civil engineer[who?] indicated that the pavement surface in the impluvia was porous, or that the non-porous stone tiles were separated by gaps significant enough to allow a substantial quantity of water caught in the basin of the impluvium to filter through the cracks and, beyond, through layers of gravel and sand into a holding chamber below ground. The circular stone opening (visible in the photograph, resembling a chimney pot) allows easy access by bucket and rope to this private, filtered and naturally cooled water supply.
Similar water supplies were found elsewhere in the public spaces of the city with their stone caps showing the wear patterns of much use (the rope wear patterns also are visible in the photograph). In wet seasons, excess water that could not pass through the filter would overflow the basin and exit the building, and any sediment or debris remaining in the surface basin could be swept away. In hot weather, water can be drawn from the cistern chamber (or fetched by slaves from supplies outside the domus) and cast into the shallow pool to evaporate and provide a cooling effect to the entire atrium. As the water evaporates, the surrounding air is cooled and becomes heavier and flows into the living spaces and is replaced by air drawn through the compluvium. The combination of the compluvium and impluvium formed an ingenious, effective and attractive manner of collecting, filtering and cooling rainwater and making it available for household use as well as providing cooling of the living spaces.