People slide down a 1,082 feet (330 metre) long inflatable water slide in Hong Kong, China August 22, 2015. (Photo by Tyrone Siu/Reuters)

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)

Wellcome trust employee Zoe Middleton poses behind an artwork entitled “My Soul” by Katharine Dowson, which consists of a laser etched lead chrystal glass formation in the shape of a brain, and was created using the artists own MRI Scan, at Wellcome Collection on March 27, 2012 in London, England. (Photo by Dan Kitwood/Getty Images)

Ferencvaros's David Siger lies on the ground to defend a free kick during the UEFA Champions League Group Stage match between Ferencvarosi TC and Juventus at Puskas Arena on November 4, 2020 in Budapest, Hungary. (Photo by Bernadett Szabo/Reuters)

The pugs dressed in Parka jackets in Sonoma County, California. (Photo by Phillip Lauer/Barcroft Media)

“Winter shapes, Moscow river”. Moscow river at this part is never covered by ice. When the temperature is very low it's possible to see a strong fog. Morning light and pipes of the oli factory creates a fantastic landscape. Photo location: Moscow, Russia. (Photo and caption by Sergey Rumyantsev/National Geographic Photo Contest)

A leaflet denouncing South Korean President Park Geun-hye lies on the ground after it was torn at a protest calling Park to step down, in Seoul, South Korea, November 19, 2016. (Photo by Kim Hong-Ji/Reuters)