money pit

Short definition from Wiktionary

Noun

1. 1
   A possession or financial commitment, especially a building or vehicle, that creates substantial ongoing expenses, especially one whose costs are considered to be unsustainable.

Proper noun

1. 1
   A complicated, seemingly man-made excavation on Oak Island in Nova Scotia, Canada, rumored to contain pirate treasure and which has been repeatedly and unsuccessfully probed at great expense.