The phrase "open manhole" refers to a hole in the ground that is usually covered, but is currently not covered or blocked. It can be dangerous because someone could accidentally fall into it. Full definition
And you watch baseball because sometimes players trip and fall down, because you want to laugh at the dumb baseball players when they disappear down open manhole covers. (sbnation.com)
Thanks Paulster2, I remember I drove my car right on to a half open manhole lid on the road at considerably high speed. (mechanics.stackexchange.com)
Open the manhole cover, and jump in. (eurogamer.net)