I'd bet some combination of ocean and environmental laws & treaties, and zealous Greenpeace types. Density isn't important - just package it in corrosion-proof

I'd bet some combination of ocean and environmental laws & treaties, and zealous Greenpeace types. Density isn't important - just package it in corrosion-proof heavy containers before tossing over the side. And do that somewhere like the Aleutian Trench - close enough to US territory to easily monitor the area, but remote / deep / inhospitable enough to make it extremely difficult for anyone to pinpoint the waste. Let alone disturb it.