Cisterns are reservoirs for holding water for thirsty cities. Istanbul has several, some open-air ones, like the Asparne cistern near the Yavuz Selim mosque where there used to be a place for cattle trade and now sports facilities for the surrounding neighbourhood (very nice place by the way). But also a number of covered ones, the most famous being the Yerebatan sarnıcı. I must have pictures of the Asparne cistern somewhere, once I find them I will include them here, in the mean time I only show these cisterns. The last one is not open for visitors, but can be seen from the side, most impressive.