[quote=@Xenonia]
Gotta be FEV.
[/quote]

Don't be silly, FEV glows bright green and is either airborne or is stored like nuclear waste! It wouldn't make a lick of sense that there could be a [i]natural deposit of FEV[/i] just lying around in some cave that the vault apparently tunneled into.

My money is on this being the same type of gak that made the Tunnelers. Vertical hasn't said too much to me on the matter.