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)

Among the fish populations that could be harmed by the Xayaburi dam in Laos is the critically endangered Mekong giant catfish, considered by the Guinness Book of World Records to be the world’s largest freshwater fish. The fish, which grows to 650 pounds and about 10 feet long, is only found in the Mekong River. It is migratory, moving between downstream habitats in Cambodia upstream to northern Thailand and Laos each year to spawn. Some experts fear the Xayaburi dam could block the migration and drive the giant catfish to extinction. (Photo by Courtesy of Zeb Hogan/University of Nevada, Reno)

Ranger trainees and trainee dogs simulate an ambush against rhino poachers at the Paramount Group Anti-Poaching training and K9 (canine) academy on November 26, 2014 in Magaliesberg, South Africa. One of the largest of its kind in Africa, the Academy will address the ever increasing need for training of Conservation Officers in anti poaching activities, wildlife contraband detection, specialist K9 solutions and Ranger K9 handler training – all of which have proven success rates in combatting and apprehending poachers and their activities. (Photo by Gianluigi Guercia/AFP Photo)

A driverless vehicle runs at Vanke's Building Research Centre testing area in Dongguan, south China's Guangdong province November 2, 2015. The country's largest property developer, China Vanke, is investing in its own robots to do certain jobs in the face of a labor shortage in the world's most populated country. This driverless car is among the robots that Vanke is aiming to bring in. (Photo by Tyrone Siu/Reuters)

An engineer (L) talks to a driver of a dump truck loaded with gold-bearing soil at the Vostochny opencast of the Olimpiada gold operation, owned by Polyus Gold International company, in Krasnoyarsk region, Eastern Siberia, Russia, June 30, 2015. Polyus Gold International is the largest gold producer in Russia and one of the top 10 gold miners globally by ounces produced, according to the official web site of the company. Olimpiada is Polyus Gold's largest operation. (Photo by Ilya Naymushin/Reuters)

A family pose for a picture as thousands of redheads fill a Dutch city Breda to celebrate International Redhead Day event in Breda, The Netherlands on Sunday, 4 September 2016. Every year natural redheads from more than 80 countries come together at “Roodharigendag” annual weekend long festival to celebrate their ginger genes. The event also holds the world record for the largest number of natural redheads being in one place. (Photo by Akmen/London News Pictures)

An echidna light sculpture eating ants is displayed during a media preview of Vivid Sydney illuminated displays at Taronga Zoo on May 24, 2016 in Sydney, Australia. Vivid is lighting up at Taronga Zoo for the first time with ten giant animal sculptures representing critical species the zoo is committed to protecting. Held annually, Vivid Sydney is the world's largest festival of light, music and ideas running for 23 days. (Photo by Cameron Spencer/Getty Images)

A hostage runs towards a police officer outside Lindt cafe, where other hostages are being held, in Martin Place in central Sydney December 15, 2014. Two more hostages have run out of the cafe at the center of a siege in Sydney, Australia's largest city, according to a Reuters witness at the site. The two women were both wearing aprons indicating they were staff at the Lindt cafe where a gunman has been holding an unknown number of hostages for several hours. Three men had earlier run out of the cafe. (Photo by Jason Reed/Reuters)