Royal Navy personnel are called out every 18 hours in a typical year to deal with unexploded bombs in the waters around the UK.
Although the precise locations of thousands of mines laid during World War Two are officially unknown, their general positions are. When mines or other unexploded items, such as torpedoes, are discovered, the Ministry of Defence calls upon the elite military Clearance Diver community to make our waters safe.
But who are the men and women who go beneath the surface, coming face to face with the danger these devices present? And what does it take to become one of the few highly trained Clearance Divers who serve in the Royal Navy?
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Here, we look at one of the Armed Forces' most dangerous roles, the typical operations divers face, and explore how safe UK waters are from WW2-era unexploded bombs.
What is a Clearance Diver?
The Royal Navy recruitment website explains the role of a Clearance Diver as "part bomb disposal master, part sub-aqua specialist."
According to the senior service, the job entails countering the international maritime terror threat as well as dealing with a "range of ordnance, from state-of-the-art mines to World War Two devices."
As there is typically just a handful of divers on board, their position is central to a ship's safety and security at sea.
Candidates for the role must prove their ability to work accurately and calmy under extreme pressure in challenging and frequently dangerous situations, with underwater conditions faced by divers ranging from freezing temperatures to operations under minimal visibility. It is a brave job for brave people.
What are the current underwater threats Clearance Divers face?
Underwater mines are not exclusive to World War Two-era leftover devices in the home waters of the British Isles. Important they may be, they form just one area of interest for the Royal Navy's elite divers.
Another critical theatre in tackling sea-based explosives is the Middle East.
Over the last few years, the security of shipping through one of the world's busiest sea lanes has resulted in extensive efforts by the Royal Navy, including tackling the threat posed by mines.
In 2020, high-profile incidents involving mine strikes by shipping cost firms responsible for vessels millions of dollars in repairs and caused severe ecological damage to maritime ecosystems.
One of those incidents occurred on 3 October 2020 off the coast of Yemen when a 100,000-tonne crude oil tanker named Syra stuck a mine resulting in a large oil spill. Experts at the Institute of Engineering and Technology said the incident was "likely to be a symptom of tensions involving the separatists Southern Transitional Council."
Instances like this have a consequence on the UK, not least because the Royal Navy maintains strategic mine hunting operations in the region, but also because 95 per cent of Britain's economic activity depends on the sea, and a vast majority of that must pass through choke points like that near Yemen.
WATCH: Royal Navy Clearance Divers in training.
How many WW2-era mines remain in UK waters?
A concerned member of the public submitted a Freedom of Information request to the Ministry of Defence in May 2016, requesting information on the locations of all known underwater mines in UK territorial waters.
The MOD investigated and said that, although the "general areas" of the British and German sea mines laid are known, the precise locations were not. Importantly, the ministry confirmed that no known unexploded mines existed, which points towards the responsive nature of how the Royal Navy deals with new finds.
Essentially, when the Ministry of Defence becomes aware of such dangers, Royal Navy Clearance Divers are deployed to make the waters safe. This process typically means moving the ordnance to a remote area and securely detonating the explosives using small military-grade charges.
Mines in the Clyde
Clearance Divers were recently called upon to deal with WW2-era explosives in the waters of the Firth of Clyde.
In December last year, members of the Northern Diving Group based at Faslane were called after fishermen trawled up what was later described by EOD personnel as a "pristine condition" mine containing around 750 pounds of explosives.
The device, placed off the island of Ailsa Craig by a German submarine 80 years earlier, was taken to a location near the Isle of Bute, where a controlled explosion was carried out.
Speaking in the aftermath of the operation, Lieutenant Commander Mark Shaw, Commanding Officer of Northern Diving Group, said that considering the mine had been underwater for such a long time, its condition was "remarkable," adding:
"Items of this size are relatively uncommon, however, the group are approaching 100 call-outs this year supporting civil authorities with all types of Explosive Ordnance Disposal, ranging from mines and torpedoes to hand grenades and improvised devices."
"On average, across the UK, Royal Navy Clearance Divers are tasked once a day for EOD assistance. This highlights the remaining presence of historic ordnance. Even small items can be unstable and present an explosive hazard; carrying-out a controlled explosion is the only safe way of dealing with them and neutralising the hazard."
Lt Cmdr Shaw's words chimed with the Royal Navy's most senior officer, the First Sea Lord, whom, in a Tweet in September, revealed that the service was responding to call-outs to the frequency of about one every 18 hours. The First Sea Lord's Twitter account also tweeted a video of a German mine being destroyed by divers in the Solent.