A gravity bong is also known sometimes as a tube, waterfall, depth charge or a sinker and is a device used for smoking weed by creating a vacuum where the suction created forces air to go through a bowl and fill a container with smoke
The Waterfall Method
This method is also known as a waterfall bong or "flood". A "waterfall" is created by having a plastic bottle with a hole at the bottom, complete with a removable bottle cap with a bowl fixed into it.
The bottle cap is taken away and the hole is covered by a finger and the bottle is filled with water. Whilst putting a flame to the bowl to light the substance the finger is taken away. Gravity makes the water flow out and causes the smoke in the chamber to vacuum out - from where you inhale.
The 3 Liter Variation
The 3 Liter Variation is much the same as the waterfall method except you use a 3 liter bottle and get a much stronger, bigger pull and hit. Great for when you want to get off your chops.
Soda Bottle Gravity Bong
This method is also known as the "third lung", the "bucket" or sometimes the "parachute". This is simplying done by taking a two liter soda bottle, emptying it and removing the bottom half of the bottle with scissors or a knife.
A big plastic bage is then put over the bottom half of the bottle and put in place with sticky tape or a rubber band. Scissors are then used to take and push through the white top of the bottle and thereby creating a small hole - just large enough to put a bowl in.
The plastic bag is push into the bottle and the substance is placed into the bowl. The cannabis is burned the smoker grabs out the plastic bag and pulls it out. This thereby makes the smoke drawn into the bottle and ready to be smoked.
If you just want to buy one than check out the vortex gravity bong.