It is the glue holding the tank to the body. Carefully pry starting at the edges to release it.
August, 6, 2012 AT 6:28 PM
Might be a little late for you but I decided not to drop a half full 40 gallon tank in mine. I pulled the carpet up over the tip of the fuel tank and used a pneumatic cold chisel to cut a rectangular opening about 6" x 9" in the floor. Twist of the access cap on the tank and pull out the fuel pump, fix or replace and put the cap back on. I used an aluminum plate from Lowes and that instant foam in a can to cover up and seal the opening. It took about a half hour. I'm really glad I did it that way because it turned out that my problem was the ECM not the fuel pump.
August, 6, 2012 AT 6:43 PM
As a disclaimer I'm sure professional car techs and the manufacturer would advise against this method and I'm sure it would void any warrantys. And obviously don't use anything that's going to create significant heat. Or Sparks when you cut, hence the cold chisel.