What if you exploded a nuclear bomb (say, the Tsar Bomba) at the bottom of the Marianas Trench?
Surprisingly littleâ€”especially compared to what would happen if you put it just under the surface.
At 53 megatons, the Tsar Bomba was the most powerful nuclear weapon ever detonated, and at 11 kilometers, the Mariana(s) Trench is the deepest part of the ocean. No underwater test has involved bombs anywhere near that size, nor depths anywhere near that deep.
The explosion at the bottom of the Mariana Trench will create a quickly-expanding spherical cavity of hot steam. To figure out how big it gets, we can try a formula from the 1971 paper Evaluation of Various Theoretical Models For Underwater Explosion:
The bubble grows to about a kilometer across in a couple of seconds. The water above bulges up, though only slightly, over a large area. Then the pressure from that six miles of water overhead causes it to collapse. Within a dozen or so seconds, the bubble shrinks to a minimum size, then â€˜bouncesâ€™ back, expanding outward again.
It goes through three or four cycles of this collapse and expansion before disintegrating into, in the words of the 1996 report, â€œa mass of turbulent warm water and explosion debris.â€ According to the report, as a result of such a deep-water closed bubble creation and dissipation, â€œno wave of any consequence will be generated.â€
That â€œturbulent warm waterâ€ is actually quite substantial. The rising column of heat creates a hot spot in the oceanâ€”many degrees warmer than the surrounding waterâ€”that lingers for some time.
Letâ€™s say that when Iâ€™m typing the above equation, the cat hops onto my desk and steps on the â€œ0â€ key, which inserts six extra zeroes.
This would really make the explosion interesting, but if you want to know more, you will have to go to XKCD to hear the rest of the story.