Word of the wise though.. Instead of cutting the bottom entirely off, just cut the little 5 feet on a 2 liter bottle off.

Having 5 small holes allows the water to leave in a more even pace, making the hit draw much, much, much thicker.

Think of it like a diffuser. If you have one hole from a down pipe the smoke still goes down. But with a diffuser you have several smaller holes. Thus allowing more smoke to be pulled, at a much more even distribution.

You don't have to cut too much off either, just the bottom flat-ish feet.

Another great idea, if you only cut the feet off, is to squeeze a ping-pong ball into the holes where the feet were. When you press the bottle down into the water the ball will get stuck at the neck of the bottle, preventing any nasty-ass bong water from getting in your mouth. It takes a little getting used to, but once you get the hang of it you'll be able to get all of the smoke into your lungs before the ping-pong ball rushes to the top of the bottle.

An alternative would be to drill/cut a moderate sized hole in the neck of the bottle, to use as a carb. Cover it while you pull up, and uncover when the bottle if fully underwater. If you let off the carb too soon you'll waste a lot of smoke because it'll shoot out of the carb. But if you let off at the right second it prevents the water from rising higher than the carb, and allows all of the smoke to be cleared with no risk of bongwaterinmouthitis.

You can tell I do my fair share of gravity bongs. Look up Blissett Bong on Google to see how to make a damned good gravity bong that filters the smoke.