full of water
also check the lower seal of your rear glass. If its leaking it would drip right into the trunk
Also it if is parked on nose down on a decline, and the hatch seal appears to be in good shape, pull the trunk seal off, run a bead of black RTV in the channel of the weather strip where it attaches to the fender.
This is mostly a camaro problem due to the fender being higher than the seal. On Firebirds, the finder is not so high.

