A girl has her helmet adjusted on the top of a tank during the Children's Day celebration at a military facility in Bangkok, Thailand January 9, 2016. Army barracks opened their doors to children to interact with their weapons and vehicles as part of the celebration. (Photo by Jorge Silva/Reuters)

A 3-month-old ethnic Tujia baby girl is pictured next to her father as they wait for a traditional ethnic Tujia wedding feast during celebrations marking the Lunar New Year in Ziqiu town, Changyang county of China’s Hubei province, February 15, 2016. (Photo by Jason Lee/Reuters)

A girl collects drinking water at Dala river outside Yangon, Myanmar March 3, 2016. Some 650 million people, or one in 10 of the world's population, have no access to safe water, putting them at risk of infectious diseases and premature death. Dirty water and poor sanitation can cause severe diarrhoeal diseases in children, killing 900 under-five a day across the world, according to United Nations estimates. (Photo by Soe Zeya Tun/Reuters)

A girl calls to her mom as she stands near a dead whale on the shore Monday morning, April 25, 2016, on the cobblestone beach at Lower Trestles on San Onofre State Beach, just south of San Clemente, Calif. (Photo by Jeff Gritchen/The Orange County Register via AP Photo)

A woman and a girl dressed in historical Red Army uniform, wait before watching the Victory Day parade, marking the 71st anniversary of the victory over Nazi Germany in World War Two, at Red Square in Moscow, Russia, May 9, 2016. (Photo by Grigory Dukor/Reuters)

A young girl takes selfie with Easter rabbit sculpture during the Easter eggs (Pysanka) and rabbit exhibition in front of St. Sophia Cathedral in Kiev, Ukraine, 05 April 2018. Ukrainians will mark Orthodox Easter on 08 April 2018, according to Julian calendar. (Photo by Sergey Dolzhenko/EPA/EFE/Rex Features/Shutterstock)

Two working girls entertaining themselves in the absence of customers at the Moonlite Bunny Ranch, a legal brothel owned by Dennis Hof, in Lyon County, one of the fews counties in the USA which permits legalized prostitution. (Photo by Stephan Gladieu/Getty Images)

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)