The Thames Estuary is a salty graveyard to hundreds of shipwrecks.

One of them, dubbed the ‘Doomsday ship’ is a Second World War cargo vessel which sunk in the murky Thames waters five miles off the coast of Southend.

The difference between this chilling wartime relic is that it happens to have 1,400 tons of TNT explosive on board (possibly a lot more). Much has been written about the SS Richard Montgomery over the years.

But here is everything you need to know about it…

What is it?

The SS Richard Montgomery was a US Liberty Ship (a class of cargo ship) weighing 7146 tons. She was built in 1943 by the St John’s River Shipbuilding Company of Jacksonville in Florida and was one of over 2700 of these mass-produced vessels built to carry vital supplies for the war effort. She was due to cross the Atlantic and help deliver munitions to help the Allied war effort but she sank in the summer of 1944. The ship was named after an Irish soldier who first served in the British Army. He later became a major general in the Continental Army during the American Revolutionary War, and he is most famous for leading the unsuccessful 1775 invasion of Canada.

Where is the wreck?

The wreck remains in shallow waters on the sandbank where she sank- across the tide close to the Medway Approach Channel, around 1.5 miles off Sheerness and 5 miles off the coast of Southend. The rusting masts of the 441ft long vessel are clearly visible above the water at all states of the tide.

When did it sink?

She began sinking on August 20, 1944. She was loaded with a cargo of thousands of tons of munitions and had joined convoy bound for the UK and then on to Cherbourg. On arrival in the Thames Estuary, the vessel was directed to anchor at Great Nore off Sheerness. The ship was to await the formation of a convoy to continue the journey across the Channel. However, on the August 20, amid stormy weather, she dragged her anchor in the shallow water and grounded on a sandbank, running east from the Isle of Grain, around 250 metres north of the Medway Approach Channel.

What happened?

Intensive efforts began by the men onboard and local dockworkers to unload her dangerous cargo. Unfortunately, by the next day, a crack appeared in the hull and the forward end began to flood. The salvage effort continued until the September 25 by which time approximately half of the payload cargo had been successfully removed. The salvage effort had to be abandoned when the vessel finally flooded completely. When the forward section flooded the vessel snapped in half.

Are there bombs and explosives still on board?

Yes there are – a lot- but there are discrepancies about just how much. Since nobody has been aboard the ship – at least not officially since it went down – and without any surviving records of what actually was removed in 1944, it’s impossible to say precisely what cargo remains. However, the Maritime and Coastguard Agency (MCA) estimates the ship likely contains a staggering assortment of more than 9,000 US-made explosives (around 1,400 tones). These include 286 giant 2,000lb ‘blockbuster’ bombs, 4,439 1,000lb devices and more than 2,500 cluster bombs. Unlike most of the other items on board, cluster bombs would have been transported with their fuses in place, leaving them more prone to detonation. Many people, however, believe there are many more explosives left on board than the MCA’s estimate- possibly up to 13,961.

Why don’t they just try to clear the wreck?

They could, but it would be incredibly risky and over the decades authorities have considered non-intervention to be the safest course of action. That became particularly true when a 1967 attempt to clear the Kielce – a smaller wrecked munitions vessel almost four miles (6.4km) out to sea – triggered an explosion that measured 4.5 on the Richter scale and damaged property in nearby Folkestone, though no injuries were reported. The MCA’s Receiver of the Wreck Alison Kentuck, who oversees management of the SS Richard Montgomery, including arranging detailed annual surveys of the site has said: “Expert advice has always been that the munitions are likely to be stable if left undisturbed. If you go and disturb them, you’re increasing the risk factor.”

Is the wreck secure?

Yes, it’s surrounded by an 800-meter exclusion, which can only be crossed by permission of the UK Home Office. The wreck is monitored by radar surveillance 24 hours a day by Medway Ports and it is surrounded by a circle of buoys to ensure that ships avoids the area. The Medway Ports operations room is within sight of the wreck and provide a first line of response to any security incursions. Every year the Maritime and Coastguard Agency commissions a survey to check how dangerous the deadly cargo is. It is kept under constant observation and any changes to the state of the wreck are investigated.

Do people try to go there though?

Not very often but there have been incidents of people going to the wreck take photos or just to have a nose around. In 2015. A paddle boarder triggered a wave of social media interest when he was pictured posing by the SS Richard Montgomery wreck. He had paddled two miles out to sea and crossed a busy shipping lane to reach the prohibited area. The wreck has also been the focus of prank callers over the years who have threatened to blow it up.

Is there a chance the explosives could go off?

Some say yes, others (including the MCA ) say, the risk of a major explosion is remote. Most agree that the bombs are relatively safe as long as they aren’t exposed to sudden shock, friction or heat. But recent MCA surveys confirm the wreck is gradually disintegrating, which could lead to a sudden collapse that triggers the sympathetic detonation of some, if not all, of the remaining explosives. If this were to happen some experts believe the consequences could be catastrophic. In 2004 the New Scientist suggested that a spontaneous detonation of the entire cargo would hurl a column of debris up to 1.8 miles into the air, causing a shock wave that would damage buildings for miles around. Before this, in 1970, a BBC news report determined that if the wreck exploded almost every window in Sheerness would be broken and buildings would be damaged by the blast. Some believe, if this were to happen, although Sheerness would get the brunt, Canvey and Southend could also be affected. However others are not so sure. Dave Welch, a former Royal Navy bomb disposal expert has advised the government on the SS Richard Montgomery’s munitions and has said he’s unconvinced by some of the wilder predictions.“ The idea that if one item goes ‘bang’ then everything will is, I think, pretty unlikely,” he says. “Unless you’ve got intimate contact between two munitions subsurface, you’ll rarely cause the other to detonate, because water is a very good mitigator. If you’ve got a 1,000lb bomb two metres from another 1,000lb bomb, the other one won’t go bang.”

Could other ships crash into the wreck?

It is a concern that the vessel could be struck by one of the many boats that pass close by every day as it lies just 200 metres from a busy shipping lane. And although the wreck is surrounded by an 875-yard (800m) exclusion zone and monitored 24 hours a day there have been at least 22 near-misses over the last few decades. In one particularly incident in May 1980, a Danish fuel tanker strayed off course in poor weather and had to take last-minute evasive action to avoid hitting the wreck.

What’s the latest on the wreck?

As well as the ongoing surveys and observations into the state of the wreck, Labour life peer Lord Harris of Haringey has been asking questions in the House of Lords about the safety of the SS Montgomery and has called for a debate on the issue. Meanwhile, it was reported last week that the Ministry of Defence is looking to hire a firm to hack off the masts of the ship which have become almost like a tourist attraction on the Thames. The action is needed as the masts are rusting and could eventually fall onto the deck below where there are bombs.