Stefan Dobrovolny of Austria and Inigo Munoz Elorza of Spain (R) stand in front of a quad bike during a simulated Mars mission on Tyrolean glaciers in Kaunertal, Austria, August 7, 2015. The Austrian Space Forum is sending some of its researchers to practice weight-less walking in spacesuits on a glacier which resembles the terrain on Mars. (Photo by Dominic Ebenbichler/Reuters)

The National September 11 Memorial & Museum (branded as 9/11 Memorial and 9/11 Memorial Museum) is the principal memorial and museum commemorating the September 11 attacks of 2001, which killed nearly 3,000 people, and the World Trade Center bombing of 1993, which killed six. The memorial is located at the World Trade Center site, on the former location of the Twin Towers, which were destroyed during the attacks. The World Trade Center Memorial Foundation was renamed the National September 11 Memorial & Museum at the World Trade Center in 2007.

Every year, in the Rostov region of Russia, a group of 40 young and not so young rural workers compete in the Bison Track Show, or more affectionately known as: Russian Flying Tractor Racing. In front of crowds numbering up to 30,000 people, a series of smoke spilling, monstrous farming machines tear round an 8km mud track, plowing through lakes and dirt mounds, their turbos screaming and tyres scrabbling to find grip.

Amazing gravity-defying photo manipulations by Ravshaniya, a photographer from Uzbekistan, teeters on the edge of levitation. Photo: Untitled. (Photo by Ravshaniya)

The shortlist for the coveted Bird Photographer of the Year awards has been announced by Nature Photographers and the British Trust for Ornithology. Here: Red-billed oxpecker on an impala, Kruger National Park, South Africa. (Photo by Edmund Aylmer/BPOTY/Cover Images/The Guardian)

A pig's head is seen in a basin after it was slaughtered it in the village of Azerany, Belarus, December 12, 2015. (Photo by Vasily Fedosenko/Reuters)

Competitors pose backstage during the Arnold Classic South America bodybuilding event in Sao Paulo, Brazil, April 22, 2017. (Photo by Nacho Doce/Reuters)

Models sit backstage during Dakar Fashion Week in the Senegalese capital, Friday June 30, 2017. The continent's fashion scene has grown steadily over the past two decades with Sub-Saharan Africa's apparel and footwear market now worth $31 billion, according to data by Euromonitor. (Photo by Finbarr O'Reilly/AP Photo)