The WWII-era SS Richard Montgomery sits just 1.5 miles from shore – and locals fear that its 1,400 tonnes of potent explosives could go off at any time. Jon Excell investigates.

To visitors unfamiliar with the hazard lurking in the waters just beyond the town’s sea wall, a terrorist mermaid is an undeniably bizarre piece of public art.

But to locals, she’s a chilling reminder of the wartime relic that some believe threatens the town and the lives of its inhabitants: the wreck of WWII-era ammunition ship the SS Richard Montgomery.

The ship lies just 1.5 miles (2.4km) from shore in the mouth of the bustling Thames estuary. Clearly visible from the land – its rusting masts rising ominously from the water – the sunken vessel contains disturbing cargo: 1,400 tonnes of high explosives which many fear could go off at any time, potentially causing one of the most devastating non-nuclear peace-time explosions ever seen.

Known semi-affectionately to locals as the “Monty”, the 441ft-long (134m) vessel was a US Liberty ship, a type of cargo ship used during World War II. It arrived off Britain’s coast in August 1944 carrying munitions to help the war effort. On 20 August, while waiting to join a convoy across the channel to France, harsh weather caused the ship to drag anchor and founder on a sand bank.

As the tide receded the vessel was left stranded. The hull’s welded plates began to crack and buckle under the weight of the explosives on board.

Local dockworkers hurriedly mounted a salvage operation. They managed to empty the rear half of the ship before finally abandoning it on 25 September, when the forward section flooded and the vessel snapped in half.

Since then, no one has been aboard the ship – at least not officially. And without any surviving records of what actually was removed in 1944, it’s impossible to say precisely what cargo remains.

However, estimates paint a worrying picture. According to a survey carried out in 2000 by the UK government’s Maritime and Coastguard Agency (MCA), the ship likely contains a staggering assortment of more than 9,000 US-made explosives.

These include 286 giant 2,000lb ‘blockbuster’ bombs, 4,439 1,000lb devices and – perhaps most worryingly of all – 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.

It seems astonishing that such hazardous cargo was abandoned so close to civilisation and in the middle of the one of the UK’s busiest shipping lanes. But in the final stages of the war, the wreck’s recovery wasn’t a priority.

In the decades that followed, authorities 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.

“Expert advice has always been that the munitions are likely to be stable if left undisturbed,” says 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 (see box out). “If you go and disturb them, you’re increasing the risk factor.”

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. Its deterioration could lead to a sudden collapse that triggers the sympathetic detonation of some, if not all, of the remaining explosives.

If this happened, the consequences could be catastrophic. Some analyses – as reported in the New Scientist in 2004 – suggest that spontaneous detonation of the entire cargo would hurl a column of debris up to 1.8 miles (three kilometres) into the air, send a tsunami barrelling up the Thames and cause a shock wave that would damage buildings for miles around, including the liquid gas containers on the nearby Isle of Grain.

It’s a scenario that keeps many, including local historian Colin Harvey, awake at night. “The remit area for the explosion would be from Margate to the centre of London,” he says. “It would level Sheerness, and a 30 or 40ft wave would breach sea defences. Sheppey’s got a population of 25,000 people. Where would they go?”

But not everyone shares this apocalyptic view.

Dave Welch is a former Royal Navy bomb disposal expert who now runs Ramora UK, an EOD (Explosive Ordnance Disposal) consultancy which carries out a large amount of work on underwater ordnance. Welch, who advised the government on the SS Richard Montgomery’s munitions, says 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. I know that for a fact – I did it last Tuesday.”

He suggests that a more likely – albeit only marginally less terrifying – scenario is the detonation of a large item initiating a ripple effect through the vessel, which would send munitions flying through the air and scatter hazardous items over a wide area.