Why does a block of plastic released under water come up to the surface of water?

Explanation

For an object immersed in water two force acts on it
`to`gravitational force which tends to pull object in downward direction
`to` buoyant force that pushes the object in upward direction
here in this case buoyant force is greater than the gravitational pull on the plastic block. This is the reason the plastic block comes up to the surface of the water as soon as it is released under water