what it smells like, more than anything else, is that sweaty smell you get from giant fat asses passing wind through their own filth. you know that smell - it was the smell the fat kid three seats up from you had in class, or the smell of particularly bad sex with a partner that's picked up a few pounds. but, it's very particularly an ass smell, and an ass smell that is drenching with sweat and excretion, for whatever reason that may happen to be. 

i'm going to guess that the sewers backed up and it interacted with the car exhaust.

it's just vicious.