A cenote is a natural phenomenon, a sinkhole in the Earth’s surface. The Yucatan Peninsula in Mexico has an estimated 7,000 cenotes because it is primarily made up of porous limestone. For millions of years, rainfall slowly ate away at the limestone and a huge system of underground caves and caverns was formed. Many filled with water from rain or from the underground water table. When the roof of a water filled cave collapses, a cenote is born. The water found in a cenote may be fresh water, salt water, or both. Structurally it may be completely open, like a lake, almost completely closed with just a small opening at the top, or somewhere in between.

Potoos (family Nyctibiidae) are a group of near passerine birds related to the nightjars and frogmouths. They are sometimes called Poor-me-ones, after their haunting calls. There are seven species in one genus, Nyctibius, in tropical Central and South America.
These are nocturnal insectivores which lack the bristles around the mouth found in the true nightjars. They hunt from a perch like a shrike or flycatcher. During the day they perch upright on tree stumps, camouflaged to look like part of the stump. The single spotted egg is laid directly on the top of a stump.

Camp director and “duck walk” inventor (Rodin Gilbert Flores, 2nd R) teaches aspiring beauty queens how to “duck walk” at a beauty boot camp in Manila in this picture taken on March 8, 2015. From wading in muddy Philippine rice paddies, former housemaid Janicel Lubina now struts down runways for the country's top designers, and is poised to be crowned among the world's most beautiful. Lubina is a star recruit in one of Manila's beauty pageant boot camps, where shy, lanky teenage girls from remote farming provinces are transformed into poised Barbie dolls who can preach about world peace in six-inch heels. (Photo by Noel Celis/AFP Photo)

Merit Prize Winner: “A Well Earned Rest in the Sahara”. This photo of Moussa Macher, our Tuareg guide, was taken at the summit of Tin-Merzouga, the largest dune (or erg) in the Tadrat region of the Sahara desert in southern Algeria. Moussa rested while waiting for us to finish our 45-minute struggle to the top. It only took ten minutes of rolling, running, and jumping to get back down. The Tadrat is part of the Tassili N'Ajjer National Park World Heritage area, famous for its red sand and engravings and rock paintings of cattle, elephants, giraffes, and rhinos that lived there when the climate was milder. Photo location: Summit of Tin-Merzouga, Tadrat, Tassili N'Ajjer National Park, Algeria. (Photo and caption by Evan Cole/National Geographic Photo Contest)

In this Wednesday, August 21, 1991 file photo, appreciative muscovites hand bread, sausages and flowers to a Soviet tank's driver who helped stop the failed hardline coup in Moscow, Russia. When a group of top Communist officials ousted Soviet leader Mikhail Gorbachev 30 years ago and flooded Moscow with tanks, the world held its breath, fearing a rollback on liberal reforms and a return to the Cold War confrontation. But the August 1991 coup collapsed in just three days, precipitating the breakup of the Soviet Union that plotters said they were trying to prevent. (Photo by Czarek Sokolowski/AP Photo/File)

These goats threw caution to the wind and scaled this Argan tree right to the very top, even balancing on the most unsturdy of branches. It looks like a bit of a baa-lancing act, but the goats hooves are perfectly adapted to climbing the trees, where they graze on the Argan fruit. Amateur photographer Burak Senbak, 51, took these photos whilst travelling through Morocco in July 2016. Burak is originally from Turkey and works as a mechanical engineer, but has pursued his passion for photography for 10 years. Intrigued by the sight of goats in a tree, Burak couldnt resist the opportunity to take some photos, and said the goats proved a perfect subject. (Photo by Burak Senbak/Caters News)

X Factor and Celebrity Big Brother starlet Chloe Khan is seen working up a sweat at her luxury home on August 30, 2016. The CBB star squeezed into a pair of super-tight leggings and a teeny crop top for a very public workout. She may be a self-made millionaire, but life wasn't always so good for Chloe Khan. The Celebrity Big Brother star, 25, has revealed she was brought up in poverty as a child in Wakefield, West Yorkshire, UK. Chloe lived with her mother on a council estate before she auditioned for the X Factor and became a playboy model. (Photo by Palace Lee)

Two of the first six “neighborhood electric vehicles” (NEV) to be leased by the U.S. Army are on display during an introductory ceremony at Fort Myer January 12, 2009 in Arlington, Virginia. The Army has leased 800 of the non-tactical NEVs with a goal of 4,000 vehicles by 2011. These NEVs are manufactured by the Global Electric Motorcars division of Chrysler Corporation. With a full eight-hour charge, the NEVs can traverse 30 miles at a top speed of 25-miles-per-hour. (Photo by Chip Somodevilla/Getty Images)