the.com/manhole cover

means A heavy metal lid set into a street or sidewalk that covers the shaft leading down to sewers, utility lines, or underground vaults.

from Plainly compound: 'man' (a human able-bodied enough to climb down) plus 'hole' (the access shaft) plus 'cover' (the lid itself) — a 19th-century industrial-age term born alongside the spread of sewer and utility systems that needed sealed, walkable entry points in city streets. The round shape got its own folklore — that it's round so it can't fall through its own opening however it's tipped, unlike a square or rectangular lid — and that explanation happens to be true engineering logic, not just a story, though rounds are also simply easier to roll into place and machine on a lathe.