Below ground nuclear geoengineering should be legally much simpler especially if it's deep enough to not significantly impact mineral rights claims.

That feels very optimistic.

Nothing nuclear is easy. People will want to make sure you are not going to contaminate the atmosphere, or the aquifers. Not just on the day of the explosion, but long term. They will want to know your plans about how you will contain the nuclear contamination for thousands of years.

And then everyone and their uncle will sue you for any property damage they think they suffered as a result of your engineering. Which can be everyone who owns real estate for hundreds of miles?

But apart from those things there are really no major legal problems with it.