Availability and management of water greatly influenced human settlement in the Maya Lowlands, and much of Mayan social innovation was centered on storing excess water for times of need. In northern Yucatan the permanent water table is sufficiently shallow that it can be accessed by natural wells known as cenotes. However, over much of the Maya Lowlands, the water table is too deep to have been available to the Maya. In response, they constructed artificial reservoirs to trap runoff. For example, Gallopin estimates that the reservoirs at Tikal (an ancient Mayan city) could have provided for the domestic needs of about 9,600 people for a period of 6 to 18 months. Even with elaborate water capture and management systems, the Maya were greatly dependent upon adequate rainfall over much of their empire and were thus susceptible to frequent or prolonged droughts that approached or exceeded the capacity of their reservoirs. In fact, evidence of droughts in the region based on studies of lake and shallow ocean sediments has led many researchers to suspect that climate was responsible for the Classic Maya collapse.