Here what happens If You Detonated a Nuclear Bomb In The Marianas Trench
One big thing to remember is that water can’t be compressed. Next, At the bottom of the trench, the water column above exerts a pressure of 1,086 bars (15,750 psi), over 1000 times the standard atmospheric pressure at sea level.
After 10 seconds, when the fireball of a 1-megaton nuclear weapon has attained its maximum size (5,700 feet across), the shock front is some 3 miles farther ahead.
At 50 seconds after the explosion, when the fireball is no longer visible, the blast wave has traveled about 12 miles. It is then traveling at 784 miles per hour, which is slightly faster than the speed of sound at sea level.
That is in open air that can be compressed.
Actually, the nuclear blast radius in water that can’t be compressed would be bigger. And in extremely High temps, and you have millions of gallons of water that is instantly converted to steam.
The Marianas trench also is a fault line.
The effects of that blast most likely will be noticed worldwide.