Final answer:
A phrase synonymous with a bottomless pit is "pouring money into a never-ending expense."
Step-by-step explanation:
A phrase synonymous with a bottomless pit is "pouring money into a never-ending expense." A bottomless pit refers to something that consumes resources endlessly, just like pouring money into an expense that seems to have no limit. This phrase emphasizes the idea of continuous spending without any visible results or progress.