From about 1650 until 1200 BC Hattuša (pronounced Hattusha) was the capital of the Hittite Empire in central Asia Minor. On the steep terrain of today's ruined city lived and worked thousands of people whose homes, cattle, tools and places of worship had to be supplied with water. The question arose regarding how water was conveyed into the large-scale ponds in the urban area. The silted East Ponds (36,000 m3) and South Ponds (20,000 m3) have been excavated since the 1980s. A supply of the large volumes of water by a long pipeline from outside the city was repeatedly discussed. Due to the topographic, hydraulic and geo-hydrological conditions however, a long distance supply would have been uneconomic and also unnecessary. Still today, many willow fountains in the region are fed by artesian groundwater. It was therefore assumed that the ponds were cut into the slope aquifers and filled during the wet season. To verify this hypothesis, groundwater monitoring stations were installed in the autumn of 2009 directly uphill of the pond banks. Observed groundwater levels 2009-2011 are low in summer but rise above the former pond surfaces during winter. The Hittites used exfiltrating groundwater also in other reservoirs avoiding hefty and strongly varying surface inflows.