Beneath the Swedish capital lies an intricate web of underground train lines. More than 90 of the 100 stations in the 110km tunnel system, sometimes referred to as “the world’s longest art gallery”, have been decorated with paintings, installations, mosaics and sculptures by 150 artists since the 1950s. After spending a couple of weeks exploring arctic Norway and Sweden, London-based travel photographer Conor MacNeill headed underground to capture images of the metro stations. Here: A rainbow arcs over a girl on the platform of Stadion station. (Photo by Conor MacNeill/The Observer)

For centuries the Highlands peoples of Papua New Guinea fought over land, women and pigs. Sorcery and battle skills could elevate a clan to Bigmanship, where the bigger the “presentation”, the bigger the man. Clans therefore would paint their bodies and create fearsome masks as part of their psy. Here: These are the terrifying tribe of “mudmen” from a remote part of Papua New Guinea. (Photo by Jeremy Hunter/Exclusivepix Media)

A spooky sight to behold! This is what a psychiatric hospital that has been abandoned for years looks like. Complete with bowling alley and a theater, the hospital shows just how much nature has weathered it over time as paint peels from the walls and the mattress gather dust. Here: This area of the hospital was last used as a day care center in the early 2000s. (Photo by Will Ellis/Caters News)

The ‘Stop the Violence’ advertising campaign by Brazilian agency Terremoto Propaganda aims to reduce the risk of accidents on the road. The visually grabbing print campaign shows a vehicle painted on the face of one person and the fist of another, drawing the comparison between what is widely known to be intentional violence, such as punching someone in the face, with what many incorrectly consider unintentional, like speeding or unsafe lane changes. Let us know what you think about this campaign in the comments below.

Michael Massaia's photographs are as nostalgic as they are disturbing. His long-exposure images capture a subject matter familiar to most. From a SpongeBob SquarePants popsicle to a Neapolitan ice cream sandwich, he frames the frozen treats most people's summer memories are made of. Yet, Massaia doesn't just realistically render his ice cream. He distorts the childhood favorites by melting them before his lens, until the pops resemble ominous pools of paint or celestial snapshots.

Volunteers transport an inflated sphere into a canoe to be lowered in MacArthur Park Lake during the installation of Portraits of Hope's exhibition “Spheres at MacArthur Park” in Los Angeles, California August 21, 2015. The installation will feature about 3,000 inflatable hand-painted spheres. (Photo by Mario Anzuoni/Reuters)

German pensioner Volker Kraft decorates an apple tree with Easter eggs in the garden of his summerhouse, in the eastern German town of Saalfeld, March 19, 2014. Each year since 1965 Volker and his wife Christa spend up to two weeks decorating the tree with their collection of 10,000 colourful hand-painted Easter eggs in time for Easter celebrations. (Photo by Fabrizio Bensch/Reuters)

“One artist doesn’t mind if people class her work as rubbish. Thats because all of her pieces are made out of junk found in charity shops, garage sales and in and around her home. Using the likes of buttons, broken jewelry and toy parts, Jane Perkins, from Exeter, UK, has recreated the famous faces of Albert Einstein, Kate Middleton, The Queen, as well as some of the worlds most iconic paintings and photographs”. – Caters News. Photo: Afghan Girl (based on photograph by Steve McCurry/National Geographic). (Photo by Jane Perkins/Caters News)