"Japanese manhole covers, which come in a variety of designs depending on locality, utility type and the manufacturer of the manhole cover, have caught the imagination of a growing number of "drainspotters" from around the world."
It may be impossible to show you all the designs all over Japan, but still we did make a try to present some of the best.
Source: 1, 2 & 3
Dog That Can Paint
Japanese Sewers Photo Gallery