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)

In this April 20, 2018, file aerial photo released by China's Xinhua News Agency, advection fog drifts across skyscrapers on the waterfront in Qingdao in eastern China's Shandong Province. Chinese leaders have long been sensitive about their communist country’s international image. Now, they are battling back, investing in diplomacy and a courtship of hearts and minds, just as the United States digs in on the Trump administration’s “America First” mindset. (Photo by Lu Hui/Xinhua via AP Photo/File)

An Iraqi medic takes a nasal swab from a woman in Iraq's central shrine city of Najaf on April 20, 2020, during the nationwide lockdown to stem the spread of the novel coronavirus. (Photo by Haidar Hamdani/AFP Photo)

Vishnu Limbachiya, a hair artist, styles the hair of a man while wearing a blindfold at a park in Ahmedabad, India, May 31, 2017. (Photo by Amit Dave/Reuters)

A child prays in front of the cenotaph dedicated to the victims of the atomic bombing at the Hiroshima Peace Memorial Park in Hiroshima, western Japan Friday, August 6, 2021. Hiroshima on Friday marked the 76th anniversary of the world's first atomic bombing of the city. (Photo by Kyodo News via AP Photo)

Freshmen take part in a military training at Chongqing City Vocational College on September 23, 2021 in Chongqing, China. (Photo by Chen Shichuan/VCG via Getty Images)

A firefighter passes through fire in a firefighting drill on December 7, 2021 in Aba Tibetan and Qiang Autonomous Prefecture, Sichuan Province of China. (Photo by Zeng Peng/VCG via Getty Images)

Senator Barack Obama (D-IL) (R) stands in front of Senator Hillary Clinton (D-NY) as they arrive for U.S. President George W. Bush's annual State of the Union speech to a joint session of Congress at the U.S. Capitol in Washington in this January 23, 2007 file photo. Hillary Clinton announced her second run for the presidency on April 12, 2015 starting her campaign as the Democrats' best hope of fending off a crowded field of lesser-known Republican rivals and retaining the White House. (Photo by Larry Downing/Reuters)