There is a curious box between the engines of an A-10. It's seems rather rudimentary, like an afterthought, although it was present on the prototype.
I've found a few cutaways but this part is hidden. I'm fairly sure that at least the front is hollow, so perhaps related to an APU. The question is, what is it for, and why, even in the context of the somewhat inelegant A-10, why so unrefined?