I'd noticed on my first day on the job that some of the CNC machines smelled a bit like feet. I suspected the coolant, but I didn't know what was in the coolant or why it smelled bad only in certain machines. Today I found out that the machines that smell like feet have bacteria growing in the coolant. Lovely. What on earth is there for bacteria to eat inside a CNC machine? There's nothing in there but tiny bits of metal and coolant. And what the hell's in the coolant? I thought the main dangers working with these types of machines were things like getting my hands cut on sharp bits of metal; I had no idea I was sticking my hands inside a bacteriological stew.
Addendum: OK, this info is a little disturbing.