I would suggest you remove a small section of the flooring in the softest spot to see what is beneath. This will tell you just how deep to set the blade on your circular saw.
Start out with a drill to ascertain you are between the floor joists! Once you find out where the joists are use a hole saw to remove a plug which will indicate the thickness of the floor.
Set the circular saw blade as close to the exact same depth of the floor as you can. Too shallow is better than too deep, you can always go over the cut.
Once the small hole is cut you may be able to determine if any floor air ducts, plumbing, or electrical wires are clear before sawing the floor. Good luck!