Remarkable story, but I have serious doubts about the long term solution in simply burying the wreck. Sediments will not remain eternally in place, and the mercury, though heavy, and likely to pool, should be removed - period. It may be a war grave, but the needs of the living should take precedence over memorializing those for whom mercury no longer poses a problem. The U-boat should be opened and the mercury salvaged, or the entire ship raised. War dead are not sacro-sanct in this type os ecological issue.