Its gone for ever, I am not sticking my hand in there.
Seriously though it is relatively easy to remove the seat from the car if you need to to retrieve something.
anything, providing its small enough to fit under the seat.
you might drop your phone under your seat
phone
your car keys
cell phone
a pistol
Loose Change
keys money
Money.
If you have lost something in the back seat like under it, i would the best thing is to remove the back seat first. The bolts of the back seat are can be found around under the cushioned area or sometimes in the boot.
A candy wrapper
A jar of black treacle.