According to the classic interpretation of the Archimedes principle, "an object
wholly or partially submerged in a liquid is subjected to an upward force equal and
opposite to the weight of the volume of water displaced".
This phenomenon is easily understood when one notes that the water in the tank
bears its own weight! The pressure which increases with depth is due to the fact
that the water above cannot sink any lower and adjusts itself automatically to
compensate for the weight of the water pushing down from above.
However, the force that the water applies to itself, is also applied (to the same
extent) to any other submerged object. Indeed, how could the water molecules
know that they are bearing the weight of water rather than that of a tennis ball?
In this way, the Archimedes principle is universal and is nothing more than
the result of forces of microscopic origin. Finally, it should be noted that it is
applicable to all fluids: liquids or gas.
When a hot-air or helium balloon floats up into the sky, it is thanks to this same
phenomenon.