Whatever the reason, reality is getting all bent out of shape. One minute you're doing your job, pressing buttons. The next thing you know you're staring into an alien world. Something huge with too many arms is taking a bite out of your partner's face. And everywhere you turn people are dying - being eaten.
Monsters are everywhere.
Monsters - there's no better word for them. You head for the surface, to get the hell away from ground zero, but the usual routes are impassable - damaged by the disaster, infested with headcrabs and houndeyes and increasingly larger and hungrier creatures. Madness is the order of the day. So much for the cavalry. You've uprooted a bunch of nasty government secrets.