The subway in Stockholm has been referred to as “the world’s longest art exhibition.” More than 150 artists have contributed to the design of 90 stations along the 68 miles of subterranean tracks. The project has been ongoing for more than 50 years! The bulk of the designs were done in the 1970s, and some stations have undergone updates since then. And each station has its own theme, colors, and atmosphere. See more gorgeous pictures at Tech Graffiti. Link

(Image credit: Alexander Dragunov)