An employee walks past a rotary dredge working at an open-cast iron ore mine of the Stoilensky mining and concentration plant (GOK), owned by the Novolipetsk (NLMK) steel mill, in the city of Stary Oskol in Belgorod region, Russia, August 4, 2015. (Photo by Maxim Shemetov/Reuters)

Bonbibi, Treehouse Point, Fall City, WA. The Bonbibi is balanced on two steel yokes. The yokes spread out the load and allow for tree growth and movement in the wind. A small deck hangs from the beams below and uses the treehouse itself to shelter guests. (Photo by Pete Nelson)

The first load of passengers queuing for the Bennie Railplane in Glasgow; the inventor George Bennie is third in the queue. The streamlined cars are self propelled, driven by air screws in front and behind, and hang from a steel girder. (Photo by J. A. Hampton/Topical Press Agency/Getty Images). 4th July 1930

A demonstration on March 15, 1922 on Palace Roof, 57th Street and 7th Avenue, showing the shellproof steel jacket. Leo Krause wearing the jacket, which weighs 12 pounds. Shots used, 38 and 45 calibre, at close range. (Photo by Bettmann Archive)

Farmers work on a farm near a construction site of new residential buildings in Shanghai, China, February 20, 2016. (Photo by Aly Song/Reuters)

A schoolgirl walks on an under-construction railway track at Navapura, on the outskirts of Ahmedabad, November 29, 2019. (Photo by Amit Dave/Reuters)

A young girl works at a construction site in Patiala district, Rajpura, India on November 20, 2019. (Photo by Saqib Majeed/Barcroft Media)

A partial solar eclipse is seen as a labourer works at a construction site in Phnom Penh, Cambodia, March 9, 2016. (Photo by Samrang Pring/Reuters)