Manholes are sealed with those flat, round, heavy metal covers you often see smack in the middle of the street. Workers enter manholes when they need to work on the sewers.
But why are they round?
If you tip a round cover on its side, it cannot fall through the hole. But if you tip a square cover on its side and turn it diagonal to the opening, it could fall through, clunking someone on the head. So it’s a safety thing.
Workers can enter sewers through manholes. Some storm sewer manhole covers have openings for rainwater like you see here. Other manhole covers are solid.