At the start of the millennium, Britain, France and Norway got together to develop a world-class system that could rescue personnel trapped in a submarine hundreds of metres below the surface.
The NATO Submarine Rescue System has just been tested the furthest north it's ever been, in the icy waters of the Arctic Circle.
However, the work doesn't end when the submariners get back to the surface. The NATO Submarine Rescue System is the best in the world because it has a portable hyperbaric chamber welded onto the ship.
No other system has anything like it.
After a casualty has been transferred straight from the back of the rescue vehicle they come into the Transfer Under Pressure System (TUP).
The TUP is a giant decompression chamber which is able to counteract the impact of nitrogen gas on the body, known as decompression illness, or the 'bends'.
When a submarine gets into distress it's likely that its inhabitants will be exposed to an increase in pressure, squeezing their oxygen levels and causing more nitrogen to form in the body.
The danger comes if a person returns to the surface too quickly and that nitrogen gas tries to escape.
To give a rescued submariner the best chance of survival they stay in the five chambers of the TUP system for hours, even days, as they are slowly brought back to normal surface pressure.
Looking after them are specially-trained submariners and divers, including Royal Navy reservists, who are on high readiness if the system is ever needed for real.
This is a tri-nation system and the roles between them are fully interchangeable. The team play to their own country's strengths, bringing different things to the table. In a time of increased global tension a system like this has its place.
Exercise Northern Sun has been a test not only of the kit in an Arctic winter but of the impact of the cold on personnel too.
However, the relationship between the countries is as strong as ever, and the system is ready to rescue trapped submariners, whenever and wherever it's needed.