It was an immersive and extensive visual world, where the main point was just to explore. It was implemented as a series of linked HyperCard stacks, each sized to fit on a floppy disk. You'd come to remember the exact points of the game that would throw up a modal dialog prompting the insertion of the next required disk.
And, it happens to have been made by the brothers Rand and Robyn Miller, who later went on to create Myst (which itself is very reminiscent of The Manhole).
Unrelatedly, I wonder if anyone in this thread remembers SuperCard, a third-party knock-off of HyperCard that offered such amazing innovations as color graphics. (I also seem to recall some kind of hack where you could use ResEdit to get colour images into HyperCard stacks even though it wasn't officially supported, but the details are fuzzy).
Two other Rand and Robyn Miller titles are there too:
I never had a Mac in that era but I'm looking forward to trying these after reading the Digital Antiquarian article about the Manhole: http://www.filfre.net/2016/10/the-manhole/
The HyperCard stack I was really interested in seeing after all these years, though, was the Mackerel Stack - alas, not there (yet)...