The Greenland ice sheet is a vast body of ice covering 660,235 sq miles, roughly 80% of the surface of Greenland. It is the second largest ice body in the world, after the Antarctic Ice Sheet. Some scientists predict that climate change may be near a "tipping point" where the entire ice sheet will melt in about 2000 years. If the entire 2,850,000 cubic kilometres (683,751 cu mi) of ice were to melt, it would lead to a global sea level rise of 7.2 m (23.6 ft).

A visitor poses with a 3D art work during a Japanese Trick Art exhibition at a shopping mall in Jakarta, Indonesia, Tuesday, December 4, 2012. (Photo by Dita Alangkara/AP Photo)

An American flag flutters on a young tree stripped of bark after a massive tornado passed through the town killing at least 116 people May 24, 2011 in Joplin, Missouri. The tornado that ripped through the town of about 50,000 people May 22, is being called the deadliest single tornado in the U.S. in 60 years. (Photo by Julie Denesha/Getty Images)

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.

Gothic rock music enthusiasts walk the streets between venues during the annual Wave Gotik music festival on June 11, 2011 in Leipzig, Germany. The festival began in the 1990s and has since grown into one of the biggest gatherings of Goth scene followers in Europe with around 20,000 participants. Many of those attending wear elaborate outfits and make-up for which they require hours of painstaking preparation and that also show a departure from the traditional black of the Goth scene. Punk remains a strong influence in today's Goth style as witnessed in Leipzig, but newer trends, with names like Cybergoth and Steampunk, have emerged that blend bold colors, Victorian fashion elegance and 19th and 20th century factory accessories into a look reminiscent of a mutated Venetian carnival. The five-day festival includes performances by around 200 bands. (Photo by Carsten Koall/Getty Images)

Islamic female students practice the ancient Thai art of “Krabi Krabong” taught as a sport at the Darunsat Wittaya school on August 16, 2011 in Saraburi, Thailand. (Photo by Paula Bronstein/Getty Images)