▁is ▁the ▁sun ken ▁part ▁of ▁the ▁at rium ▁in ▁a ▁Greek ▁or ▁Roman ▁house ▁( dom us ). ▁Design ed ▁to ▁carry ▁away ▁the ▁rain water ▁coming ▁through ▁the ▁compl u vi um ▁of ▁the ▁roof , ▁it ▁is ▁usually ▁made ▁of ▁mar ble ▁and ▁placed ▁about ▁ 3 0   cm ▁below ▁the ▁floor ▁of ▁the ▁at rium ▁and ▁em pt ied ▁into ▁a ▁sub floor ▁c is tern . ▁ ▁In spe ction ▁( without ▁exc av ation ) ▁of ▁impl uv ia ▁in ▁Pa est um , ▁Pom pe ii ▁and ▁Rome ▁indicated ▁that ▁the ▁pav ement ▁surface ▁in ▁the ▁impl uv ia ▁was ▁por ous , ▁or ▁that ▁the ▁non - por ous ▁stone ▁t iles ▁were ▁separated ▁by ▁g aps ▁significant ▁enough ▁to ▁allow ▁a ▁substantial ▁quantity ▁of ▁water ▁caught ▁in ▁the ▁bas in ▁of ▁the ▁impl u vi um ▁to ▁filter ▁through ▁the ▁cra cks ▁and , ▁beyond , ▁through ▁layers ▁of ▁gra vel ▁and ▁sand ▁into ▁a ▁holding ▁chamber ▁below ▁ground . ▁The ▁circular ▁stone ▁opening ▁( visible ▁in ▁the ▁photograph , ▁res emb ling ▁a ▁chim ney ▁pot ) ▁allows ▁easy ▁access ▁by ▁bucket ▁and ▁ro pe ▁to ▁this ▁private , ▁filtered ▁and ▁naturally ▁co o led ▁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 ▁ro pe ▁wear ▁patterns ▁also ▁are ▁visible ▁in ▁the ▁photograph ). ▁In ▁wet ▁seasons , ▁excess ▁water