table in the middle of the peninsula, with saline water underlying the whole of the peninsula. Types In 1936, a simple morphometry-based classification system for cenotes was presented. Cenotes-cántaro (Jug or pit cenotes) are those with a surface connection narrower than the diameter of the water body; Cenotes-cilíndricos (Cylinder cenotes) are those with strictly vertical walls; Cenotes-aguadas (Basin cenotes) are those with shallow water basins; and Grutas (Cave cenotes) are those having a horizontal entrance with dry sections. The classification scheme was based on morphometric observations above the water table, and therefore incompletely reflects the processes by which the cenotes formed and the inherent hydrogeochemical relationship with the underlying flooded cave networks, which were only discovered in the 1980s and later with the initiation of cave diving exploration. Flora and fauna Flora and fauna are generally scarcer than in the open ocean; however, marine animals do thrive in caves. In caverns, one can spot mojarras, mollies, guppies, catfish, small eels and frogs. In the most secluded and darker cenotes, the fauna has evolved to resemble those of many cave-dwelling species. For example, many animals don't have pigmentation and are often blind, so they are equipped with long feelers to find food and make their way around in the dark. Chicxulub crater Although cenotes are found widely throughout much of the Yucatán Peninsula, a higher-density circular alignment of cenotes overlies the measured rim of the Chicxulub crater. This crater structure, identified from the alignment of cenotes, but also subsequently mapped using geophysical methods (including gravity mapping) and also drilled into with core recovery, has been dated to the boundary between the Cretaceous and Paleogene geologic periods, 66 million years ago. This meteorite impact at the Cretaceous–Paleogene boundary is therefore associated with the mass extinction of the dinosaurs and is also known as the Cretaceous–Paleogene extinction event. Archaeology and anthropology In 2001–2002 expeditions led by Arturo H. González and Carmen Rojas Sandoval in the Yucatán discovered three human skeletons; one of them, Eve of Naharon, was carbon-dated to be 13,600 years old. In March 2008, three members of the Proyecto Espeleológico de Tulum and Global Underwater Explorers dive team, Alex Alvarez, Franco Attolini, and Alberto Nava, explored a section of Sistema Aktun Hu (part of Sistema Sac Actun) known as the pit Hoyo Negro. At a depth of the divers located the remains of a mastodon and a human skull (at )