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.