Subject: How do submarines sink and then float again.

I have to do a science fair in my school right now, and this was one of my 
ideas. I really need to know how submarines sink and float for part of my 
research. Also if you could supply me with other people and resources I could 
call, then it would help me a great deal. Last, for my experiment, i plan on 
haveing an empty soda bottle in a tub of water. It will have 2 holes in it, one 
on the side and another on the top. I'll pump air into the bottle and force any 
water out through the hole in the side. The problem is that i need something to 
plug the hole on the side so that the air doens't escape while im pumping air 
in. Unless i get a really powerful pump, i dont know what to do. please help me 
with these things. thanks.

Re: How do submarines sink and then float again.