Douglas Harper's Etymology Dictionary
money-pit
"edifice or project requiring constant outlay of cash with little to show for it," 1986 (year of a movie of the same name); see money (n.) + pit (n.). Before that (1930s), it was used for the shaft on Oak Island, Nova Scotia, that supposedly leads to treasure buried by Capt. Kidd or some other pirate. "Whether that name refers to the treasure or the several million dollars spent trying to get the treasure out is unclear." [Popular Mechanics, Sept. 1976]