The fourth annual BigPicture Natural World Photography Competition aims to celebrate the diversity of life on Earth, and encourages people to protect and conserve it. Here: “The Salmon Catchers”. Terrestrial Wildlife. To capture this view of a mother grizzly bear and her cub, photographer Peter Mather set up a camera trap on a log that he knew the bears tended to traverse while fishing for salmon, in the Yukon River watershed in Canada. (Photo by Peter Mather/BigPicture Natural World Photography Competition 2017)

Hooded demonstrators vandalise a shop window in Mexico City, during the commemoration of the 50th anniversary of the 1968 Tlatelolco student massacre, on October 2, 2018. Fifty years ago, Mexican troops opened fire on student demonstrators, killing hundreds just days before Mexico City hosted the 1968 Olympics – one of the darkest episodes in a year of global turbulence. (Photo by Ronaldo Schemidt/AFP Photo)

In northwest Russia, in a small village called Alekhovshchina, Nadia Sablin's aunts spend the warmer months together in the family home and live as the family has always lived, chopping wood to heat the house and making their own clothes. Sablin's book of photographs, “Aunties: The Seven Summers of Alevtina and Ludmila”, is published by Duke University Press. Here: “Two-Handed Saw, 2014”. (Photo by Nadia Sablin)

A woman lays in a puddle of tomato juice during the annual “Tomatina” tomato fight fiesta in the village of Bunol, 50 kilometers outside Valencia, Spain, Wednesday, August 27, 2014. The streets of an eastern Spanish town are awash with red pulp as thousands of people pelt each other with tomatoes in the annual “Tomatina” battle that has become a major tourist attraction. At the annual fiesta in Bunol on Wednesday, trucks dumped 125 tons of ripe tomatoes for some 22,000 participants, many from abroad to throw during the hour-long morning festivities. (Photo by Alberto Saiz/AP Photo)

Belgian animal sculptor Emmanuel Janssens Casteels works at his desk next to a replica of a Smilodon, an extinct genus of machairodont felid, in his workshop in Prayssas December 3, 2014. Casteels' company OPHYS specializes in life-size reconstructions and the sculpting or moulding reproductions of animals and hyper-realist fossils. His reconstructions are exhibited in museums and theme parks. (Photo by Regis Duvignau/Reuters)

A horse pulls a driving school car as other drivers push their vehicles behind during a demonstration by driving instructors in Belgrade, September 25, 1992. Due to gasoline shortage in the Yugoslav capital, driving instructors are staying away from work. (Photo by AP Photo)

Audrey Hepburn cooking at home, 1954. Colorized by Dana Keller.

A novice monk holds a book during an inauguration ceremony at Jogye temple in Seoul, May 11, 2015. Ten children on Monday were given the opportunity to experience life as Buddhist monks by staying at the temple until Buddha's birthday in two weeks. The writing on the book reads, “Novice Monk Prayer”. (Photo by Thomas Peter/Reuters)