The Stockholm underground system, or Tunnelbana, was carved out of the bedrock beneath the Swedish capital, beginning in the 1950s. In order to mitigate the rather scary effect of this organic architecture, caused by the exposed, unsculptured bedrock, the authorities commissioned what has become known as “the world’s longest art gallery”. More than 90 of the 100 or so stations have some form of art installation. These images show the most striking and colourful stations.