Just an interesting thing I stumbled on whilst searching for a recording of the THX Deep Note - this is an article on how it was created, and how to recreate a reasonable facsimile of it:

It would be an interesting project to improve on this, perhaps by using real orchestral samples! (the original appears to use a single sample of a cello, if I'm not mistaken - it would be interesting to use multiple samples, each optimised for a particular register, and carefully morph between them for the long crescendo)