present at various depths. The whole of the Yucatán Peninsula is an unconfined flat lying karst landscape. Sinkholes, known locally as cenotes, are widespread in the northern lowlands. According to the Alvarez hypothesis, the mass extinction of the non-avian dinosaurs at the transition from the Cretaceous to the Paleogene Period, the Cretaceous–Paleogene boundary (K–Pg boundary), 66 million years ago was caused by an asteroid impact somewhere in the greater Caribbean Basin. The deeply buried Chicxulub crater is centered off the north coast of the peninsula near the town of Chicxulub. The now-famous "Ring of Cenotes," a geologic structure composed of sinkholes arranged in a semi-circle, outlines one of the shock-waves from this impact event in the approximately 66-million-year-old rock. The existence of the crater has been supported by evidence including the aforementioned "Ring of Cenotes", as well as the presence of impact debris such as shocked quartz and tektites, a type of glass formed during meteorite impacts. The Arrowsmith Bank is a submerged bank located off the northeastern end of the peninsula. Climate The peninsula has a tropical climate, which ranges from semi-arid in the northwest to humid in the south. Average annual rainfall ranges from less than in the driest parts of the northwest up to in the Petén Basin to the south. Rainfall varies seasonally, with August and September generally the wettest months. Like much of the Caribbean, the peninsula lies within the Atlantic Hurricane Belt, and with its almost uniformly flat terrain it is vulnerable to these large storms coming from the east, and the area has been devastated by many hurricanes, such as Hurricane Gilbert, Hurricane Emily, Hurricane Wilma, and Hurricane Dean. Strong storms called nortes can quickly descend on the Yucatán Peninsula any time of year. Although these storms pummel the area with heavy rains and high winds, they tend to be short-lived, clearing after about an hour. The average percentage of days with rain per month ranges from a monthly low of 7% in April to a high of 25% in October. Breezes can have a cooling effect, humidity is generally high, particularly in the remaining rainforest areas. Water resources Due to the extreme karst nature of the whole peninsula, the northern half is devoid of aboveground rivers. Where lakes and swamps are present, the water is marshy and generally unpotable. Due to its coastal location, the whole of the peninsula is underlain by