Why is the law of conservation of mass true

The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction. This is TRUE in the sense that it describes and generalizes the observation, that a sample of matter will not change in mass, even if it undergoes a chemical reaction. It holds true because naturally occurring elements are very stable at the conditions found on Earth. Atoms are not converted to other elements during chemical reactions. Because of this, atoms that make up living and nonliving matter are very old. The atom itself is neither created nor destroyed but go through a series repeatedly among chemical compounds. 

Because the dept in the ocean