Cartman: What are we, the Goonies?
The Goonies is a 1985 movie about a group of underpriviledged kids who are about to lose their housing project. They find a treasure map that promises to solve their problems.
The map takes them underground, just as the South Park boys are in this episode. Mr. Garrison snorkeling gives Cartman the feeling that the search for Mr. Hankey is much like a treasure hunt.