According to National Whatever Day, April 11 is National Submarine Day. Some sites say it is actually supposed to be March 17th and others say it is April 17th. No matter what day it is, submarines are so amazing.
A submarine works because it has ballast tanks. To sink it fills the tanks with water and to surface it fills the tanks with air. Last summer I made a submarine out of a water bottle. It was a great time.
It took us a couple of days to make it work but it was really neat. All the kids liked to take turns pumping air to make it rise.
(In the video I said the rock is ballast but really the water is ballast.)
We got the idea from education.com