Manhole covers are such an underrated Japan thing, glad you're into it! If you ever want to share your shots, we've actually got a community page on Hey Japan for exactly this: www.hey-japan.com/community/ma... - would love to see what you've collected so far.
Manhole Designs | Hey Japan!Decorative Japanese manhole covers — community collection of ward designs and iconic landmark covers.www.hey-japan.com