Wow.
There are dump sites in waters around the world, according to a 2009 Annual Report to Congress from the Department of Defense Environmental Programs. In waters bordering the United States, including Hawaii and Alaska, tens of thousands of ammunition and explosives, as well as millions of pounds of chemical munitions lurk on the sea floor.
There are at least 5,400 tons of sulfur mustard in waters off the Atlantic coast and 9,100 tons off the Pacific coast.
The Department of Defense estimates that a total of 17,000 tons of sulfur mustard exist in United States waters, when the 2,300 tons buried off the coast of Hawaii and the 57 tons in Alaskan waters are included.
Although the number of dumped weapons may seem ludicrous in the environmentally conscious 21st century, it was considered the safest option for munitions disposal until 1970, according to a 2009 Department of Defense report.
The vastness of the ocean and its inaccessibility at certain depths seemed like the ideal trashcan for the volume of dangerous weapons left after two world wars. It was believed that seawater would dilute toxic chemicals, and that weapons deep underwater would be permanently unavailable to the hands of foreign powers.