A bottlenose dolphin places a marking device on an exercise sea mine (Picture: US Department of Defense).
A Beluga whale found by Norwegian fishermen could belong to the Russian Navy, according to reports.
It was discovered wearing a harness, on which the Norwegian Directorate of Fisheries said "Equipment St Petersburg" was written, which also had a mount for an action camera.
Experts believe Belugas are among the mammals which can be used by militaries in a variety of roles.
While the use of dogs and other animals in militaries across the globe is well known, considerably less is written about the use of dolphins.
The US Navy began working with bottlenose dolphins and California sea lions in 1960 to help with mine detection and the design of new submarines and underwater weapons.
Tests had been undertaken with more than 19 species of marine mammals, including some sharks and birds, to determine which would be most suited to the work needed doing.
In the end, it was the dolphins' highly-evolved biosonar, which made them helpful for finding underwater mines, and the sea lions' impeccable underwater vision, which made them able to detect enemy swimmers, that saw them come out on top.
And it being the height of the Cold War, the Soviet Navy was not about to risk being left behind.
Retired Colonel Viktor Baranets, who observed military dolphin training in the Soviet and post-Soviet eras, said the mammals were part of the broader Cold War arms race between the US and the USSR. He told AFP: "Americans looked into this first.
"But when Soviet intelligence found out the tasks the US dolphins were completing in the 1960s, the defence ministry at the time decided to address this issue."
In 1965 the Soviet Navy opened a research facility at Kazachya Bukhta, near Sevastopol, to explore the military uses of marine mammals. This was then passed to the Ukrainian Navy after the fall of the Soviet Union, marking the end of the Soviet military dolphin program.
Baranets says the training centre was severely neglected in the coming years, with reports in 2000 that its dolphins had been sold to Iran, with the chief trainer carrying on his research at their new oceanarium.
The Ukrainian navy re-established the centre in 2012, but it came back under Russian control after the country annexed Ukraine's Crimea region in March 2014.
What are they used for?
During the Cold War dolphins were used to spot suspicious objects or individuals near harbours and ships, as well as detecting submarines or underwater mines.
Baranets says Soviet combat dolphins were trained to plant explosives on enemy vessels and could detect abandoned torpedoes and sunken ships in the Black Sea.
Russia's defence ministry made some unusual headlines in 2016, meanwhile, after paying £18,000 for five bottlenose dolphins.
America, for its part, trains dolphins as well as sea lions under the US Navy Marine Mammal Program, based in San Diego, California. The US Navy spent $14 million in 2007 on marine mammal research and training programmes.
Military dolphins continue to be used to locate underwater mines, as well as for object recovery and the rescue of lost naval swimmers.
Due to the secrecy surrounding the use of dolphins in the military, there is also a number of rumours of different uses.
It has been claimed military dolphins have been trained to lay underwater mines, locate enemy fighters, or even to seek and destroy submarines using kamikaze methods.
It has even been speculated that dolphins have been used to carry poison darts and sonar jamming devices, with the potential for combat between different countries' marine mammals also discussed.
The US Navy, for its part, denies having ever trained its marine mammals to injure humans or to carry weapons capable of destroying ships.
It is known, however, to have used them during both Gulf Wars, while sea lions were deployed to Bahrain in 2003 to support Operation Enduring Freedom after the September 11 attacks.
How are they trained?
Training of dolphins follows much the same pattern as that of police and hunting dogs.
They are trained to detect underwater mines and enemy swimmers and then report back to their handlers, who give them rewards such as fish on correct completion of a task.
A full-time staff of veterinarians, veterinarian technicians and highly-trained marine biologists care for the US Navy's marine mammals, with doctors and staff on call around the clock in case the animals need care.
Dolphins and sea lions are kept healthy and fit for duty with routine physicals, with nutrition oversight and extensive data collection and management also used to keep them in good shape.