Saul's "crack in Inglewood" joke was great, but didn't make me laugh as much as his Ghosbusters comment a couple of episodes back. Love Saul.
The cracks... Hmm. Intriguing. What can cause such huge cracks?
-underground explosion
-massive earthquake
-gigantic. living thing emerging from the earth
-gigantic living hulk-like thing smashing the ground
-Satan and the Archangel Gabriel in the movies The Prophecy

Beyond that, I'm stumped. Well, there were those alien robots from War of the Worlds, but I doubt that.
The gas obviously takes a while to turn you into a zombie... Or it needs to hit the bloodstream in high doses to do so, otherwise Vic would have gone brain-eater on us already. Thus, the idea of a virus can be dismissed. Definitely seems more like a Biological/chemical agent.

Tanya and Saul immune. Could it have to do with their race? Just thinking that there aren't many African-American characters and the two that there are have been exposed in some way and not turned. Just a thought.

All in all, I'll stick to the giant flying squid theory... Seems legit.