This 3,000-km² region is located in northern Quintana Roo, Mexico, in the northeastern corner of the Yucatan Peninsula. The boundary of this region runs from the north coast 75 km southward, is 40 km wide, and is defined primarily by its unique water resources. It is characterized by a karstic limestone platform that contains only a few small lakes and no surface rivers. Despite this, it has the most abundant water sources of the entire peninsula. First, it receives the greatest annual rainfall of the northern Maya lowlands (up to 2,000 mm), which recharges an underground aquifer and contributes to widening a series of north-south fractures within the porous limestone shelf, known as the Holbox fracture zone. These fractures contain freshwater wetlands, which the ancient Maya exploited for fish, periphyton (a nutrient-rich algae used as fertilizer), and propagating food plants (see also Subsistence). The region is dotted with thousands of cenotes (karstic sinkholes) that provided direct access to the water table, and caves are common, which the ancient Maya believed represented entrances to the underworld (see also Groundwater/Water Table). Due to their symbolic importance, some regional caves contain architecture (i.e., altars, small pyramids), petroglyphs (symbolic designs incised into the cave walls; see also Iconography), and remnants of ceramic vessels that would have been used to collect sacred water from pools found deep within the caverns.