Ocean disposal of radioactive waste
From 1946 through 1993, thirteen countries used ocean disposal or ocean dumping as a method to dispose of nuclear/radioactive waste with an approximation of 200,000 tons sourcing mainly from the medical, research and nuclear industry. The waste materials included both liquids and solids housed in various containers, as well as reactor vessels, with and without spent or damaged nuclear fuel.
Source: Wikipedia — Ocean disposal of radioactive waste (CC BY-SA 4.0)