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. You've opened the portal to another world, and an alien light comes shining through. Some things you just have to see for yourself. In Counter-Strike, as in Team Fortress 2, the player experiences what a trained counterterrorist unit or terrorist unit experiences.