Drone warfare is a hot topic and one that defence experts have been discussing for decades. Indeed, UAVs have been a reality for almost the same amount of time.
But conversations about this element of military operations have typically always been about drones that fly, not those that swim. That is, until now.
Whereas the Royal Air Force has been using drone technology to engage targets from afar for years, the Royal Navy has yet to use the technology for combat purposes. But all that is set to change.
In 2020, the MOD announced it had awarded a defence contract to British-based MSubs ltd to produce what is reported to be the world's largest underwater drone.
The Portsmouth-located business specialises in building military and scientific submersibles and will, as part of its contract with the MOD, deliver an XLUUV (extra-large unmanned underwater vehicle) expected to measure around 100 feet in length.
Here are some of the possibilities this new technology could bring and how it could impact future operations across combat and surveillance.
Speaking in Pearl Harbour, Hawaii, during a recent visit, Admiral Sir Tony Radakin described the Royal Navy's intention to protect the sites of underwater war graves.
These sites, the final resting place of thousands of British and Commonwealth sailors and which many consider sacred, have attracted criminal looters keen to strip the sunken ships of their precious metals.
To help protect these important sites, some media reports suggested that drone technology, including devices disguised as manta rays, could be deployed, technology it said could also be used to 'spy on enemy submarines.'
But what underwater or on surface drone capability does the Royal Navy already possess?
Exercise Unmanned Warrior
In 2016, the Royal Navy hosted Exercise Unmanned Warrior. At the time, it was described as a "record-breaking" display of unmanned and autonomous vehicles.
Unmanned Warrior comprised 50+ autonomous systems and saw the Royal Navy bring together more than 40 different organisations and partners. Back then, the Royal Navy said that the exercise demonstrated "the power of technology to make the world a safer place."
Alongside underwater drones, like other sections of Britain's military, the Royal Navy possesses unmanned aerial drones and drones that operate on the sea's surface.
In July, the Royal Marines Commandos were supported by "drone swarms" in a UK Armed Forces first. The Royal Navy said that the drones "dived, sailed and flew" together for the first time in the military's exercising history.
Will Britain's New Underwater Drones Carry Weapons?
MSubs Ltd, a defence supplier, recently awarded contracts to build autonomous submarines for the Royal Navy, described an XLUUV like the one it is producing for the MOD on its website.
It names the XLUUV as Mantra S201 and explains its weight as 8.9 tonnes, and says it has an operational depth of "up to 305m".
"Dived performance is maximised by attention to streamlining of the outer hull and by the use of advanced motor technology to directly drive the main propeller. Surface running is possible with one crew member on deck and hatch closed, at speeds up to 6kn in modest sea states, but the vessel's primary mode of deployment is by crane-in." -MSubs Ltd.
Some reports suggested that the new unmanned underwater vehicle would have the capability to carry weapons, and have a range of around 3,450 miles, or 3,000 nautical miles.
What Else Can Drones Be Used For Under The Sea?
The new Royal Navy drone could, if reports about its weapon capabilities are followed through, arguably provide a potential threat to any current or future adversary to the United Kingdom.
But drones can also provide equally vital surveillance capabilities, which are not necessarily limited to front line defence.
First Sea Lord Admiral Sir Tony Radakin has been reported as intimating that the Royal Navy's aspirations would be to examine ways in which technology would be able to cover war graves and large maritime protected areas such as with the use of unnmanned underwater vehicles.
Listening To The Sound Of The Sea
Environmental Acoustic Recording Systems (known as EARS) can be placed deep in the ocean, which then records the world's normal sounds beneath the sea.
According to Forbes, "a navy which knows what the ambient noise in a specific corner of ocean should sound like, all year around, knows when it sounds different. Sound is the cornerstone to detection, and stealth, in submarine warfare."
Yet, EARS are somewhat standard and certainly not something new. In terms of their autonomous nature, they are required to be fixed in a specific location by being anchored to the seabed and have to be recovered at a later date manually. So, the technology is nothing ground-breaking or futuristic.
However, there has been an evolution in this area by advanced militaries around the globe. In 2020, it was reported that sophisticated "spy drones" had been discovered by Chinese fishermen and handed over to authorities.
The devices were said to have been operating in international waters, but "in China's backyard". A Forbes article said their "role [was] likely to be intelligence gathering."
"The information they collect could include measuring the depth, noise, salinity and currents. This seemingly mundane data could provide submariners with a tactical advantage in future operations, making them better informed about local conditions. Which is why navies invest so much in these activities. And why they are often conducted discretely, or even covertly."
When Were Underwater Drones Invented?
The United States Navy began using drones in the 1960s, although they were first trialled a decade earlier.
Their original intended use was to detect lost equipment and explore lost wrecks on the seabed in Pearl Harbour or the South Pacific.
The US Navy used drone technology to help locate a lost nuclear bomb off the coast of Spain in 1966 following an accident involving a B-52 Bomber.
Another, perhaps more famous example of a sea drone detecting something important came in 1985 when the wreck of Titanic was discovered at a depth of 12,500 feet in the North Atlantic Ocean.