Basically, regular submarines blow compressed air into the ballast tanks when they need to surface. Because the Trieste goes so deep, compressed air wouldn't work. It uses fine steel shot instead. But at this point, to make it neutrally buoyant, one needs a large amount of material that is only slightly lighter than water. It also has to be mostly incompressible, so aviation gasoline it is. The Trieste effectively becomes an underwater airship. Or a watership, if you will.