Inset tiles in the Mackintosh building, Glasgow School Of Art, Glasgow, UK - perhaps the greatest building in the UK. The two back staircases, completed in the second phase of construction, perversely get darker and more dungeon-like as you ascend, and lighter and brighter on the lower floors. The architect Charles Rennie Mackintosh inset geometric patterns of coloured tiles into the wall on each landing. Over the years, many people have tried to interpret them as some sort of cipher, perhaps some message he left. If there is a code, no one has yet worked out what it means.