A girl uses her mobile phone during the evening at the National Park in Khartoum, Sudan, April 24, 2015. (Photo by Mohamed Nureldin Abdallah/Reuters)

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 June 29, 2016 file photo, boats docked at Central Marine in Stuart, Fla., are surrounded by blue green algae. The 153-mile-long Indian River Lagoon has been plagued by harmful algae blooms. Water quality testing data analyzed by the AP showed the average phosphorous level – a byproduct of fertilizers and human waste that algae thrive on, rose nearly 75 percent between 2000 and 2016. (Photo by Greg Lovett/The Palm Beach Post via AP Photo)

Over 30 Douglas C-47 Skytrains (Dakotas) making final preparations for flight formation practice before the 75th D-Day Anniversary flight to Normandy in Duxford, England on June 4, 2019. (Photo by UK Air Force Images)

A woman takes a picture of a statue of a child wearing what appears to be a hazardous material suit in Fukushima, Japan on August 14, 2018. (Photo by Kwiyeon Ha/Reuters)

Randy Orton is seen in action against Rey Mysterio during the single match of WWE “Crown Jewel” World Cup 2018 tournament at King Saud University stadium in Riyadh, Saudi Arabia on November 2, 2018. (Photo by Faisal al Nasser/Reuters)

Vehicles pass through the heavy fog and air pollution that cover Skopje valley in the early morning, in Skopje, The Former Yugoslav Republic of Macedonia, 14 December 2017. According to media reports, Skopje is considered one of the most polluted cities in the world. (Photo by Georgi Licovski/EPA/EFE)

The sunken ferry Sewol sits on a semi-submersible ship during its salvage operations at the sea off Jindo, South Korea, in this handout picture provided by the Ministry of Oceans and Fisheries and released by Yonhap on March 26, 2017. (Photo by Reuters/Yonhap/The Ministry of Oceans and Fisheries)