I've been pretty interested in manhole covers for the last week or so. I read about them in the brilliant The 99% Invisible City and since then, well, I've gone down the manhole, as it were.
The 99% Invisible City talks about the Manhoru movement – a Japanese trend for decorative manhole designs displaying "location-specific motifs" and everything from Hello Kitty to elements of folklore. Back in the eighties, the original impetus for this stuff was to raise awareness of subsurface utilities at a time when only half of Japanese households were connected to municipal sewers. Since then it has become a thing of joy and creativity in its own right.
My sister was once very excited about a manhole cover in Canterbury that had a spelling mistake on it. But there is so much more to be intrigued by. Outside of Japan, I was fascinated to discover that the cross-hatching patterns a lot of manhole covers have is to aid traction. Yet that is just the start.