Ngorongoro Crater (Tanzania). At 610m deep and 260 sq km, this is the largest unflooded caldera in the world. A blue-green vision from above it's a haven for engangered wildlife and Maasai livestock. The crater was formed three million years ago when a giant volcano, which could have been as high as Kilimanjaro, exploded and collapsed. The caldera formed the concentric fractures in the crust cracked down to a magma reservoir deep underground. (Photo by John Bryant/Getty Images)

Descendants of American Southerners Wearing Confederate-era dresses and uniforms dance during a party to celebrate the 150th anniversary of the end of the American Civil War in Santa Barbara d'Oeste, Brazil, Sunday, April 26, 2015. For many of the residents of Santa Barbara d'Oeste and neighboring Americana, in Brazil's southeastern Sao Paulo state, having Confederate ancestry is a point of pride and is celebrated in high style at the annual “Festa dos Confederados”, or “Confederates Party” in Portuguese. (Photo by Andre Penner/AP Photo)

Women sing and dance as children gather in the background during the so-called parade of children's troops in Rostov-on-Don, southern Russia, May 14, 2015. About 500 children aged from 4 to 10, members of youth military patriotic clubs, honor guards and paratroopers took part in the event, which was organized under the guidance of Commander of Southern Military District Colonel-General Alexander Galkin and was held to honor World War Two veterans and mark the 70th anniversary of the victory over Nazi Germany, according to local media. (Photo by Eduard Korniyenko/Reuters)

A Russian-made rifle, carried by this woman when she was captured in Nearby Brush, is slung around her neck by South Vietnamese soldiers before they began their interrogation of her as Viet Cong suspect near AP La Ghi in Vietnam, August 25, 1965. The girl behind the woman, also was captured in the brush. (Photo by AP Photo)

Port staff check the damaged bridge cranes in Kaohsiung, southeast China's Taiwan on September 15, 2016. Kaohsiung Port saw facilities damage due to the effect of Typhoon Meranti. (Photo by Johnson Liu/Xinhua via ZUMA Wire)

A layer of morning mist still covers the fields as the sun rises into the morning sky over Hohenfelden, Thuringia state, Germany, early 07 September 2016. Meteorologists predict the weather to become warm and sunny in the region with temperatures reaching around 256 degrees Celsius. (Photo by Martin Schutt/EPA)

Revelers of the “Societe des Champs Elysee” launch fireworks before boarding the Rampart-St. Claude street car line, which just opened last fall, to commemorate the official start of Mardi Gras season, in New Orleans, Friday, January 6, 2017. (Photo by Gerald Herbert/AP Photo)

Forest guards chase away a rhino that strayed into a residential area after floods forced animals to escape the Kaziranga National park in Nagaon district, in the northeastern state of Assam, August 15, 2017. (Photo by Anuwar Hazarika/Reuters)