Whether an object floats or sinks (buoyancy) depends on the density of the object and the density of the fluid. 

How does the amount of air an object contains affect its buoyancy? 

Open the valve to let air out of the vessel. Close the valve to pump air in. 

Can you make the submersible move up and down the tube? 

How do submarines work? 

How can you make a sinking object float?

 

The more air there is in the bell, the lighter it is, because the air displaced the heavier water. The bell becomes more buoyant. By opening the valve, air will be let out of the vessel and it will begin to sink. By closing the valve air will stop escaping. With the valve closed you can also pump more air into the vessel to make it float again.

Forces in Action