I don't know the details of this, but a guy I worked with once told me he did work detecting underground structures using gravity measurements. I guess there's

I don't know the details of this, but a guy I worked with once told me he did work detecting underground structures using gravity measurements. I guess there's lots of relatively affordable equipment to so something like this. https://archive.epa.gov/esd/archive-geophysics/web/html/gravity_methods.html https://archive.epa.gov/esd/archive-geophysics/web/html/grav...