Beyond

My guess is the metal in question is acting as an antenna and picking up radio signals from the Southern Scadrians.

Another possibility for ettmetal is uranium, which, in its metal form is quite exothermic chemically with water. It would make sense for it to be a power source and be able to be in a bomb capable of destroying a city. Being the last natural element on the periodic table, that could make it the "final metal" referenced by Miles.