You're referring to the wreck of the Richard Montgomery, a Liberty Ship which sank in the Thames Estuary with a cargo of munitions. The Richard Montgomery had been loaded with some 7,000 tons of munitions, prior to joining Convoy HX301 from New York to the UK. The munitions were ultimately destined for Cherbourg, but there was a delay between the arrival of HX301 in the UK and the departure of the next Cherbourg convoy. On the 20th August 1944, the Richard Montgomery was ordered to anchor in the Nore anchorage off Sheerness until the Cherbourg convoy was ready. However, the water where she anchored was too shallow, and, as the tide fell, her anchor dragged and she ran aground on a sandbank. As she sat on the sandbank, her hull started to break forwards of the bridge; this was a notable weak spot in the Liberty Ship design. On the 23rd, a crew of stevedores was organised to remove her cargo, as the damage that had already been done made it unlikely that she could be refloated from the sandbank. The next day, though, her bow holds started to flood, making it hard to remove the cargo from them. Work continued as the ship continued to deteriorate; on the 8th September, the Richard Montgomery's back broke, splitting her into two parts. As the risk increased, the Royal Navy was brought into take over. Ultimately, though, the worsening weather and the increased difficulty of accessing the forward holds - and ensuing risk of explosion - caused the abandonment of the operation. Her three forward holds, containing about half the munitions aboard, remained largely full. While the holds likely contain 3,500 tons of munitions, only 1,400 tons of this is explosives. The remainder of the weight is in bomb casings and the like. Since the abandonment of the salvage operations, there have been no further attempts to remove explosives from the wreck.

There are several reasons for this inactivity. The main one is that, for much of the wreck's history, there has been little risk of an explosion. The main contents of the forward holds were aircraft bombs, filled with TNT. TNT is a very stable explosive, especially when it is stored at a low temperature which varies exteremely slowly. This happens to be very similar to the conditions experienced at the bottom of the Thames Estuary. Other explosives, or those stored in less appropriate conditions, will become more volatile over time, as their chemical components decay or change state, making them more sensitive to shock, friction or other stimuli. TNT, though, stored well, will not explode without a significant shock, usually provided by another 'primary' explosive. Most of the bombs on the ship were carried unfused, so these primary explosive are not present. Those few that were carried fused were designed such that the fuse, and the primary explosive it contained, was not inserted into the bomb until it had left an aircraft. None of the fuses carried were designed to be submerged, so they have likely filled with water, neutralising their explosive content. With this in mind, the British government felt that the bombs were safe as long as they werre not disturbed. The risk of a ship ramming the wreck was thought to be low. The wreck was on a sandbank outside of the usual shipping channels, and its masts provided a useful visual reference for the position of the sandbank. The wreck was (and still is) regularly surveyed, to ensure that it is not falling apart in such a way as to increase the risk of explosion.

Even so, the explosives could still be salvaged. However, to do so greatly increases the risk of explosion. The forward holds are not easily accessible by divers, being covered by the decks and hatch covers. To get into them would require either partially dismantling the wreck, or sending divers in through relatively small gaps. The problem with the latter is clear. The bombs are not easily man-portable, and trying to move them around without heavy equipment has a large chance of an accident. Dismantling the wreck, meanwhile, means either using cutting equipment or blasting charges to cut away the parts of the wreck that make accessing the holds difficult. This risks dropping parts of the wreck onto the bombs, or setting them off with the cutting tools or charges. The difficulty of salvaging explosives from a wreck was made clear by a 1967 incident involving the wreck of the SS Kielce. This was a Polish ship chartered by the US Military, which sank with a full cargo of explosives after a 1946 collision. In the 1950s, the wreck was discovered off the town of Folkestone, and in 1966, a contract offered for her salvage, to ease access to the port. A local salvage company was given the contract. The salvagers began to detonate small charges around the wreck, to break up some of the hull plating. However, their third charge set off the explosives in the wreck, with an estimated yield of 2,000 tons of TNT. Fortunately, the wreck was far enough away from Folkestone to do little damage to the town. Repeating this with the Richard Montgomery, though, would risk serious damage to the town of Sheerness, as the wreck is much closer to the shore.