I love this project so much my ninth grade students and I made a giant (6' tall) Cartesian diver exhibit for our local children's museum. Of course, there weren't any soda bottles that big. We fabricated it using a rigid Plexiglas tube. For pressure, we pumped in air with a bicycle pump. It worked great, and we had fun.
When I do this project with groups of kids, I ask what happens if a stone drops in water. Every kid knows that it sinks. And they all know what happens when a piece of foam is dropped in water. Then I ask what would happen if one ton of foam were dropped into the ocean. Here there is less certainty, and it gets to the heart of what density is all about. Density isn't just about weight. It is just as much about how much space something takes up. Density is the ratio of weight divided by volume. If the density is greater than water, it sinks. If the density is less than water, it is buoyant.
I get a thrill out of seeing air compress. My kids say I'm easily amused, but after many years of knowing in theory that air was compressed into scuba tanks it was exciting to see it happen with my own eyes.
Here I will stop and refer you to a great web page about Cartesian divers. It is by a science teacher who not only did a phenomenal job of researching the history on the device, but who provides a great example of how to teach. Read about things like how student simulated a zero-gravity environment (jumping off a diving board into a pool) to test a theory about how the diver works! Set the "view-text size" on "largest" and check out this page: http://www.ed.uiuc.edu/courses/CI241-science-Sp95/resources/philoToy/philoToy.html
another kind of submarine project http://www.phptr.com/articles/article.asp?p=413663&seqNum=2&rl=1
Here is a picture of beautiful hand-blown glass divers sent to me by Eugenia from Mexico, along with an explanation:
When I found about the Cartesian Divers, I remembered mine. But these are made of blown glass. I'm sending you 2 pictures. These I bought in Puebla, Mex. 20 years ago. I remember my father putting one of his in a white wine bottle and pushing the cork in so the diver would go down, and he controled the depht with the cork. My brothes and me were little children and it was like magic. (There were no recyclable PET bottles to use). So my father's divers must had been 50 years old or so. We called them "tiny divers". My father was an accountant so didn't explain the science part, but I remember him as a science man, he had a lot of nice things, like the drinking bird (Photo) -(I would love to hear your explanation of how that works) and many amasing science toys.
Now I'm teaching my 9 years old son about the wonderfull science things that happend around us and your page makes it easier and fun.