The Bestiary

[Big D]

Oooh...I think I missed that. Im still in the process of reading it. Could you explain? Is it to do with those bug things that ruin metal working on the ground? Thats the only thing I can think of.

I don't remember precisely, since it's been a while, but it goes something like this:
The explanation is that there's a kind of slightly corrosive vapor that occurs naturally, emerging from the ground and possibly a by-product of Mist. This is harmless to living things, and doesn't affect metal or structures, but in the long-term it plays hell with machinery of any kind. It only appears at low altitudes, though, so airships are virtually immune.