It smells like time has stopped, as if nature herself is being denied... It smells like how one would imagine Frankenstein's creation to smell, it's a low musky invasive smell, which leaves you feeling sullied and sick. You have only to take a wiff to understand and to know... and that wiff with haunt you for the rest of your years. *shudder* I still loved cutting open the cat though.
Formaldehyde gas is usually dissolved in water, where it's called formalin. It's pungent with an unpleasant odor. You'd have to just smell it -- there isn't an easy way to describe an odor in words.