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)

A pilot steers his helicopter at night over Rio de Janeiro, Brazil, less than two weeks before the start of the Rio 2016 Olympic Games, July 23, 2016. Picture taken July 23, 2016. (Photo by Pawel Kopczynski/Reuters)

Selfie picture shows Swiss pioneer Bertrand Piccard during the last leg of the round the world trip with Solar Impulse 2 over the Arab peninsula July 25, 2016. (Photo by Jean Revillard/Bertrand Piccard/Reuters/SI2)

Lulu, her Playa name, falls into a foam pit as approximately 70,000 people from all over the world gather for the 30th annual Burning Man arts and music festival in the Black Rock Desert of Nevada, U.S. September 1, 2016. (Photo by Jim Urquhart/Reuters)

Peruvian shamans holding a figure of a Nino Jesus (Child Jesus) and a snake perform a ritual at the Rimac river to fight the negative effects of the Nino weather phenomena over Nature, in Lima, October 1, 2015. (Photo by Mariana Bazo/Reuters)

These pictures show the beautiful ruins of abandoned temples. The incredible shots, taken at temples in Angkor, Cambodia, show the striking yet haunting temples as they crumble to ruins. While they may have lost some of their former glory, the stunning temples show the beauty of the Cambodian landscape through the historic architecture. Here: Sun rises over the 5 towers of Angkor. (Photo by Alex Teuscher/Caters News)

British rally driver Kris Meeke jumps his Citroen over a brow during the fifth special stage of the World Rally Championship (WRC) Rally of Australia, near Bellingen on the New South Wales central coast on September 12, 2014. (Photo by William West/AFP Photo)

Crampus from Netherworld Haunted House with Amber Thompson, spa staff, at Bliss Spa at W Atlanta Downtown hotel on Thursday, October 7, 2014. A self-guided, dark attraction known for its over-the-top special effects, intense make-up, costuming and stunt actors as well as unique monsters, unusual themes and chilling detail, Netherworld Haunted House in Atlanta is taking over the Halloween holiday this season for its 18th season. (Photo by Hyosub Shin/AJC)