The Romans are generally credited as being the first concrete engineers, but archaeological evidence says otherwise. Archaeologists have found a type of concrete dating to 6500 B.C., when stone-age Syrians used permanent fire pits for heating and cooking. These fire pits, built from area limestone, showed a primitive form of calcining on the exterior faces of the limestone rocks that lined the fire pits and lead to the accidental discovery of lime as a fundamental building material. The newly discovered technology was widely used in Syria, as central lime-burning kilns were constructed to supply mortar for rubble-wall house construction, concrete floors, and waterproofing cisterns