Metal production doesn't surprise me since I would imagine mining of some kind would be around in an underground civilisation and presumably they could use the lava/magma/whatever as a heat source. They probably could have used that as barter currency until paper currency showed up, at least. (Possibly also some kind of glasswork, if they had anything that could be melted down to make glass or a glass equivalent? Idk, that one's reaching a bit.)

And yeah, piracy from Haixing wouldn't surprise me. Smuggle down those resources, yo. XD Seize the means of production or whatever. >.>