The "Big Dig" was originally a $2.5B project that just kept growing and growing. The latest count has it up to $14.6B but we're talking Massachusetts politics here so there can never be an honest accounting of all the expenses.
I never read the whole story, only having heard parts of it. I know there were some problems building an interstate tunnel underneath Boston but why would it be 12 billions dollars worth? Unless 90% of the federal money is lost to corruption.