Nearly every station in the London Underground contains an enamel plaque depicting a labyrinth. The collection were installed in 2013 by artist Mark Wallinger to mark the system’s 150th anniversary. Each of the 270 black and white designs is unique to its location, and all of them are posted in publicly accessible locations, so visitors can examine them directly, tracing the path with a finger. They’re numbered according to the route taken by the contestants in a 2009 Guinness World Records challenge to visit all stations in the system in the fastest time.

A list of all 270 labyrinths is here.