The Manhole is a computer game, released in 1988, and notable for being a free exploration game, having a point and click interface, and having no "enemies". The game was also important, after the fact, because its designers, The Rand Brothers, went on to produce Myst.
The game starts at the titular manhole, which can be opened to produce a beanstalk, a la Jack and the Beanstalk. Following it down leads to a ship, following it up leads to a tower. You can also knock on the door of a fire hydrant. By pointing and clicking, the player (if such a term even applies), can navigate at will. Just as the game has no challenge in game play, it also has a very whimsical tone, with many references to tropes of childrens books and fairy tales.
It is hard to review a twenty year old video game, especially one designed to be experimental. Since the game presents no challenges, and the players movement is unfettered by anything but their observational powers, it is pretty easy to "play through" the game in an hour or so, leaving very little replay value. Also, most of what the game does could be done today with some clip art,photoshop or the gimp, and some basic html. Its hard to imagine this was once a piece of commercial software, but then people also used to pay for operating systems.
But if we take away our jaded perspective, it was an interesting idea, and if expanded upon, and not put in terms of forcible whimsy, the idea behind the game was captivating: an interlocking and circular world where nooks and cranies could contain any number of secret connections. There have been many other pieces of great computer software based on the same premise. Such as, of course, the site you are reading now. Perhaps redoing and expanding the concept of The Manhole with the e2 engine could be someone's stroke of genius.