SHEERNESS, England — In the mouth of the River Thames, just downstream from London, a team of elite marine salvors has been busy trying to stabilize one of the world’s most dangerous shipwrecks.

The SS Richard Montgomery, a U.S. military cargo ship, dragged anchor, broke her back on a sandbar, flooded and sank in late summer 1944, a few months after D-Day, with 1,500 tons of explosives still in the holds.