Visitors walk past the fully equipped dining table inside the “Crazy House”, which is completely built upside-down, in the village of Affoldern near the Edersee lake, May 7, 2014. Three friends came up with the idea to build the tourist attraction, which cost about 200,000 euros and took some six weeks to complete. (Photo by Kai Pfaffenbach/Reuters)

Completion is near on Monaco's tallest skyscraper ever, and its first new one since the 1980s. Fueled by a luxury-housing boom in the tax-haven nation, the Odeon Tower's so-called Sky Penthouse is expected to sell for more than any other apartment in the world has ever sold before. Photo: The five-story Sky Penthouse is expected to be complete in September 2015. The water slide extends from a dance floor in the residence down to a private infinity pool. (Photo by Tour Odeon)

A 16-meter-high snowman is nearly completed by the Songhua River in Harbin City, northeast China's Heilongjiang Province on December 7, 2022. (Photo by Rex Features/Shutterstock)

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.

A tattoo of Captain America drawn by artist Sean Karon on the leg of client Ron Raucci at the Hampton Roads Tattoo Festival in Virginia, on March 2, 2012. The tattoo was completed from start to finish in one five-and-a-half hour session and won the prestigious “tattoo of the day” contest. (Photo by Jason Reed/Reuters)

Havilland Mosquito KA 114 performs a low pass during an airshow commemorating the completion of its rebuild on September 29, 2012 in Ardmore, New Zealand. The plane was restored by Warbird Restorations at Ardmore Aerodrome and is the only flying Mosquito in the world. (Photo by Simon Watts)

An employee stands on the step of The Venice Simplon-Orient-Express at Istanbul Station in Istanbul, on August 31, 2022. The Venice Simplon Orient-Express luxury train arrives in Istanbul, completing its annual voyage along a mythical route that takes it across Europe from Paris. (Photo by Yasin Akgul/AFP Photo)

“The World Stands on its Head” (“Die Welt Steht Kopf”) House on the Baltic Sea Island of Usedom stands nearly completed on September 3, 2008 in Trassenheide, Germany. The upside down house, complete with upside down interior furnishings, is the brainchild of Klaudiusz Golos and Sebastian Mikiciuk, and will become a local tourist attraction. (Photo by Sean Gallup/Getty Images)