The explosion is a simultaneous detonation only.
Assume starting level is ~500 feet above sea level.
By Mantle, I mean the outer edge. The explosion doesn't necessarily need to reach the very core (and would be exponentially greater I believe) but must expose the outer mantle to the air.
How big of a crater would such an explosion make, supposing the optimal placement and shaping was used in the explosion?
Assume the geological makeup is similar to Earth's, and the area is mostly bedrock.
The mantle is the same size as the Earth's, as is the total size of the planet (it's an alternate dimension setting).
The bombs are each approximately fifteen (15) megatons each (Castle Bravos).
Thanks in advance!
EDIT: Added the megaton specification.