Grooves in manhole covers are necessary to provide traction in all kinds of weather to the vehicles above. But there’s no reason those grooves can’t spell out fun. Japanese manhole cover designs range from sublimely patterned to the surreal and fully illustrative. I haven’t been able to find any background information in English, but it seems there are not only different designs for the different types of conduits they cover, but each city has its own designs celebrating some signature feature of the town. Some are even maps to the local landmarks. Here are some sites with pictures:
On a related note, see also this portable Japanese emergency toilet design that fits over a manhole in case a natural disaster. Screw that port-a-john business. When the City collapses, I want one of these handy.
I took the photo above near the main train station in Osaka.