In this Wednesday, July 13, 2016, handout image provided by Solar Impulse, the Solar Impulse 2 flying over the pyramids, Egypt Cairo. The experimental solar-powered airplane has arrived in Egypt as part of its global voyage. (Photo by Jean Revillard, Rezo via the AP Photo)

Željava Air Base, situated on the border between Croatia and Bosnia and Herzegovina under Plješevica Mountain, near the city of Bihać, Bosnia, was the largest underground airport and military air base in the former Yugoslavia, and one of the largest in Europe. (Photo by Thomas Windisch/Exclusivepix Media)

A man jokingly poses with a new installation of British graffiti artist Banksy's art in New York, October 20, 2013. (Photo by Carlo Allegri/Reuters)

A performer takes part in the opening of the Temple Fair, part of Chinese New Year celebrations at Ditan Park, also known as the Temple of Earth, in Beijing, February 18, 2015. (Photo by Kim Kyung-Hoon/Reuters)

Yang Juan, an employee at Goopal Group, takes a nap in her seat after lunch, in Beijing, China, April 21, 2016. Office workers sleeping on the job is a common sight in China, where a surplus of cheap labour can lead to downtime at work. But in China's technology sector, where business is growing faster than many start-up firms can hire new staff, workers burn the midnight oil to meet deadlines and compete with their rivals. Some companies provide sleeping areas and beds for workers to rest during late nights. (Photo by Jason Lee/Reuters)

A Russian police officer detains a teenager during rally protesting retirement age hikes in St. Petersburg, Russia, Sunday, September 9, 2018. A government plan to increase the age for collecting state pensions brought protests across Russia's 11 time zones on Sunday even though the opposition leader who called them was in jail. Nearly 300 people were reported arrested. (Photo by Roman Pimenov/Interpress photo via AP Photo)

Long row of shiny new Flying Fortresses, part of huge reserves being built up in Britain for D-Day, stands by to be flown to combat units as replacements, May 25, 1944. (Photo by AP Photo)

An injured Palestinian demonstrator is helped during a protest against the planned evictions of Palestinian families in the Sheikh Jarrah neighborhood of east Jerusalem, Saturday, May 8, 2021. (Photo by Oded Balilty/AP Photo)