Submarines are a remarkable invention! Unlike a ship, a sub can control its buoyancy by adjusting the amount of air or water in its ballast tank, allowing the sub to float either on top of or below the water. The sub's enclosed environment is engineered for the crew to live and work comfortably with all the air and fresh water they need. Incredible!

Here's how it works:

• When the ballast tanks are full of air, the sub weighs less than the volume of water it displaces and it floats.
  
  
• When the ballast tanks are filled with water, the sub weighs more than the water it displaces and it sinks.
  
  
• To rise out of the water, the sub reduces its weight by pushing compressed air into the ballast tanks, forcing the water out and lifting the sub up to the surface.
  
  
• To move beneath the water, the amount of water in the ballast tanks is made equal to the weight of water it is displacing, allowing the sub to propel itself through the water.