Seems like it depends on what it is as far as what to do about it. Maybe you’ve got pets that added something to the vents, or children that thought it would be a cool thing to try out for an open pit potty — maybe with a pioneer spirit in mind :-)
Seems like you could pour some enzyme solution down there to hit or miss but if you get it too wet, you’ll have mold problems also.
There are people who come and clean out the air vents, but they do have a fee that would not be fun to pay.
That’s a tough one — you can’t stick something down there to see if you hit anything to find out what it is? or maybe a hanger or wire hooked to pull anything back up?
Good luck! :-)