well in the wake of WWII the US had many routes for the US war machine supplies and one such was near Puerto Rico however the Germans new this and U-Boats used the area as hunting ground it probably sunk with a ship and got swept off then drifted through the ocean till it washed a shore
For many decades US navy has used puertorican beaches as junk yards and practice range areas near civilian populations. That tank was the bulls eye for fighter jets. Many "casualties" had sufer from the out-of-the-charts cancer levels in the region or even direct hit from missiles. Safe to say that enviromental laws, aparently, don't apply to US goverment.