A body, water tank, ice, sub or anything displaces its own weight in water but only floats when its volume\'s density is less than that of water. In ice the density is about 0.9; ie, a cubic meter of ice weighs about 900 Kilograms and displaces that amount of water or 0.9 cubic meters leaving 0.1 cubic meter above the surface. In a hollow heavy walled structure such as a water tank or submarine, its weight must increase to match and only very slightly less than the weight of the water it\'s volume displaces. Once it is under there is very little, almost no or no buoyancy - depth is controlled by motion and front and back planes. No forward motion with plane action = Rise to surface or SINK! So taking on water and blowing it out with very light air (even when compressed) allows the sub to go up on and down under the surface. This however varies over the length of the mission in that as you deplete food and drinking water, fuel oil and torpedoes the amount of water taken on to submerge increases and that much more must be blown out when surfacing. Controlling this bouancy is tricky and requires skill and training.

Modern submarines displace 10 to 20 times what the old ones did; that\'s why they are so big.

EXPERIMENT: Take a glass jar with tightly sealing lid and a bucket full of water. Start with any empty jar floating and then gradually add water and note the changes. Try the same with coins or other metal items. HAVE FUN!