These so called "black coffin" it was said that it is a metal box with serial numbers. These boxes were dropped by U.S. Navy in the Philippines seas and it is said to contain PLUTONIUM.
The US military has no history on the exact locations of these items and words of giving rewards are out there to whoever can give locations of even just one of these items.
to make it authentic you must get the serial# of the coffin by stencil underwater. But be careful in doing so, do not ever try to puncture or damage the box. It is said that engines of small fishing boats just turn-off whenever it gets near the box,it will take a few minutes to re-start it and starts only when it is far from the box.
Maybe some fisherman have found it, maybe some have tried to break it underwater or maybe some have DIED of unknown illness, without knowing they have already been exposed to this highly toxic silvery radioactive metallic element.