

Absolutely nothing.
For reference, here’s footage of a 5 Megaton nuclear device being detonated underground. No global eruptions, and hard for humans to even feel further from the epicenter - although, seismic monitoring by precision instruments is a known way of monitoring for secret underground nuclear tests.
For further reference, the 2011 Tohoku earthquake off the coast of Japan - the one that unleashed powerful tsunamis - released a staggering 3.9x10^22 Joules, or around 9.3 million megatons of TNT. While this did cause local devastation, it also failed to meet Project Vulcan’s goal.


“What can the harvest hope for, if not the care of the reaper man?”