That’s what I love about the Japanese, attention to things that seems insignificant to the rest of the world… even though it’s such a prominent feature in our cities…
Really cool!

also here: http://www.artlib.ru/index.php?id=11&idp=0&fp=2&uid=185&idg=0&user_serie=2390

A manhole cover is a removable plate forming the lid over the opening of a manhole, to prevent anyone from falling in and to keep unauthorized persons out.Since the era of ancient Rome, sewer grates made from stone have been used to keep people from falling into the sewage and to catch anything large that might otherwise fall in. Manhole covers usually weigh more than 50 kg, partly because the weight keeps them in place when traffic passes over them, and partly because they are often made out of cast iron, sometimes with infills of concrete. This makes them inexpensive, strong, and heavy. A manhole cover sits on metal base, with a smaller inset rim which fits the cover. The base and cover are sometimes called “castings,” because they are made by a casting process.