For her water-supply, and especially for her drinking water, Athens depends, has always depended, not on her rivers but her wells. In describing the Enneakrounos Pausanias[264] says, ‘There are wells throughout all the city, but this is the only spring.’ His statement as regards the spring is not strictly correct. Besides Kallirrhoë the ancient city possessed two natural springs, and these both on the Acropolis itself, the Klepsydra at the North-West corner and the spring in the precinct of Asklepios on the South slope. About the wells he is right. The plain on which Athens stood was, owing to its geological structure, amply supplied with wells. Its uppermost stratum is of calcareous stone, the material of which the hills of Lykabettos, of the Mouseion, and the Acropolis are all formed. Through this stratum rain can freely filter. But beneath this calcareous layer is a second stratum of slate and marl; this is practically impermeable, and here water collects into wells. [Illustration: FIG. 35.] Wells, then, occur sporadically all over Athens and the Athenian plain, but nowhere in such abundance as in the district under discussion[265]. The Pnyx and the Mouseion on the one side, the Areopagos and Acropolis on the other form, as will readily be seen by reference to Fig. 46, a sort of trough, in which both rain and subterranean water are caught and must necessarily accumulate. As the ground slopes towards the North and the West the water accumulated cannot make its way towards the Ilissos. Its only outlet is the narrow and inadequate passage between the Pnyx and the Areopagos to the Eridanos. It is not surprising that, though the district lies high above the bed of the Eridanos, it was somewhat marshy. That its watery character was early turned to account and led to a dense population is shown by the fact that no less than 100 wells have been sunk within its narrow limits. These wells will be seen dotted about all over the plan in Fig. 35. These wells for subterranean water are frequently reinforced by cisterns for collecting rain-water. The cisterns are easily distinguished from the wells by the fact that they are lined with cement. Sometimes an old well which has presumably run dry has been turned by a coat of cement into a cistern. It is very remarkable that, long before the days of Peisistratos, elaborate systems existed for collecting water, in wells, cisterns, and conduits; one canal extended as far as the Odeion of Herodes Atticus, and followed a course almost coincident with that of Peisistratos, which it long preceded. Its complex of wells is clearly seen at T in Fig. 35, a little to the North of the ‘Branch Conduit to Koile.’ It is beside our purpose to examine in detail the artificial water-supply[266] of the district before the time of Peisistratos. That such a system existed is worth noting, because it shows that the district is a good site for the _Limnae_, and also that it was from early days thickly populated.