Re: Deep Underground Military Bases - Minus Serpents...
There is a genetic table that specifies which species can be sucessfully interbred with others. It has to do with the relative gene count of the male compared with the female. The only drawback is that the offspring are sterile, as in the mule.
This is rumoured to be one of the programs at Broom Lake, or Area 51, so the alien cover story fits nicely with the perception control, or psychological camouflage necessary to maintain operational security there.