For reference this question considers the movie The Core, so if you haven't seen it, please see the linked plot overview for more details, but a brief summary is, the world will end unless the Earth's core can be restarted, which will take lots of money, but must be kept secret to avoid panic.
In the film, they say "15 billion" dollars which is not a lot of money by state standards. Out of curiosity, this asks what mechanisms a liberal democratic government could use if it was "a big" amount of money by state standards. In other words, something you couldn't easily hide in a defense budget, for instance.
In short: how would they accomplish to allot the funding a very large project for that even though the federal budget was (just as today) negotiated in Congress and published to the general public?
For more context, I'm wondering, isn't there some sort of account book where you could discover a 4 trillion dollar hole?
Specifically, how would a liberal democratic government, with a legislative branch, conceal appropriations for such large amounts of funding?
The question is not asking how it could be internally justified, which is straightforward as in: the world must be saved, but in order to save it, a project must run which has to be kept secret (otherwise the world will be severely damaged by panic, riots, economic depression, etc). This question is asking, how it could be implemented to secretly (covertly and maintaining the security classification of the project) get such a large amount of funding and execute the project.