An underwater explosion, also known as an UNDEX, is an explosion

Explosion

An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...

 beneath the surface of water. The type of explosion may be chemical

Explosive material

An explosive material, also called an explosive, is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure...

 or nuclear

Nuclear explosive

A nuclear explosive is an explosive device that derives its energy from nuclear reactions. Almost all nuclear explosive devices that have been designed and produced are nuclear weapons intended for warfare....

. They are categorized in accordance with their depth beneath the water's surface, because this has a strong influence on their effects.
The effects of an underwater explosion depend on a number of things, including distance from the explosion, the energy of the explosion, the depth of the explosion, and the depth of the water.

Underwater explosions are categorized by the depth of the explosion.