The ship sank 75 years ago after unloading over 2/3rds of the 6 tonnes munitions from holds 1 and 2, and has been decaying ever since. The top of the mast to the base of the ship is about 100 feet and the remaining munitions that were mainly unfused aircraft bombs totalling about 1,400 tonnes of TNT in holds 3, 4 & 5. It’s a bit strange as hold 3 broke before unloading and flooded and the three holds total about 40% of the capacity. It sits about 70 feet of water ranging from about 60 to 80 feet. The top of the holds are probably around about 40 feet above the keel and the decks being around 15 feet above that. So the closest is probably within 5 feet at very low tide when it’s most likely to collapse. The welded steel construction means that there is about 4,000 fragmented tonnes of it above the explosives. It’s not exactly the same but consider a 14 pounder cannon has a range of 4.5 miles, but that is using propellant rather than high explosives that have a lot more power. Nobody knows the effect of deterioration but I could image a small bolt making 10 miles if it happens in a worst-case scenario, maybe as far as Eastchurch or Thorpe Bay. If the people in the area were evacuated for the day, and there was a remote controlled explosion at high tide with the blast going downwards from the deck it would probably be a lot safer.
That said, TNT is normally a very safe compound by itself and hard to detonate, even after many years.
You wouldn’t be able to fire it off by hitting it with a hammer for instance, so if that’s all that was there, then it wouldn’t be a problem. But with large-scale metal collapse there is the possibility of a strong enough shock wave being produced by a sudden crush and compression of material, especially if another type of bomb that was not removed went off or a possibly in a superstructure collapse. In which case it’s likely, one up all up.
You also have to consider that if a chance of explosion is the reason why it’s safer to do nothing or investigate it, then there is a substantial risk of explosion and vital information is missing. If it won’t explode, then there is no explosion risk in investigating it, especially if there is the possibility it can be done remotely.
If you want the scale of the problem, the ship carries an estimated 1,400 tonnes of TNT, the bombs that the Luftwaffe usually dropped were 250kg bombs (5,600 of those), the V2 rockets that hit the UK were 1 tonne in size (1,400 of those), and Little boy that exploded 1/3rd of a mile above Hiroshima was supposed to be equivalent to about 15,300 tonnes of TNT (so Montgomery is 9% of it’s size).
If the reports are to be believed most of the 1,400 tonnes of munitions is unfused sealed TNT bombs. You could hit it with a hammer and it would not go off as the shockwave needed to ignite it wouldn’t be there, so there would be next to no chance of explosion. Even after 75 years it’s still very stable stuff even mixed with sea water. If I suspect what they fail to mention is what is also there that was in hold 3 that broke up. TNT has a power about 20,000 times that of petrol and is toxic to marine life, but only gets dangerous in a combination of sea and sun.
If there’s no chance of explosion it can be safely inspected, if it can’t be safely inspected there’s a chance of explosion. What they are worrying about is a similar thing that happened to a Polish ship Kielce in 1967 that was in much deeper water and had a much smaller cargo, causing an effect that was recorded at 4.5 on the Richter scale 3-4 miles away. There have been two similar events elsewhere in the world that have killed hundreds.