money pit

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A possession or financial commitment that creates substantial ongoing expenses, especially one whose costs are considered to be unsustainable.
• noun sometimes capitalized Long-standing nickname of 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.