Trainspotter? No, drainspotter actually.

Ever hungry to feed an autistic need to spot and document I have stumbled across the perfect pastime: drainspotting. While psychogeographing through the urban landscape you can plot your route by (photographically) collecting manhole covers. As these sites show, there are some real treasures to found underfoot wherever your travels take you. I will now judge a city and a culture by how well it satisfies the drainage aesthetic. The manhole cover on the left is in Germany by the way.
Sewers of the World Unite, Covers to Discover & Drainspotting.