The Dunker is a world-class facility for essential Underwater Escape Training (UET), simulating how to escape from a helicopter underwater.
Those undergoing the Dunker are briefed by staff, before strapping themselves into a mock cockpit which is then submerged in a swimming pool.
Forces News joined Apache pilots as they took part in the essential training - which they must complete every two years - at RNAS Yeovilton.
Senior Dunker instructor Billy Gill said: "The Dunker is a way of us being able to train military personnel – predominantly aircrew, special forces, people who fly often over water at night – what to do in a crash.
"We call it a ditch, but a ditch is a crash on water."
There are a variety of simulators used to represent the aircraft that the Navy, Army and RAF use, including the cockpits of Apaches, Lynx, Merlin and Sea Kings.
Every effort is made to make it as realistic as possible says Senior Instructor Gill.
"It's so beneficial to be confident and understand the surroundings," he said.
"If I was to put you in a different seat from your normal aircraft, you would struggle because your memory isn't used to different handles, harnesses, all that sort of stuff."
A key part of the training is on using a STASS – a short-term air supply system – designed to give a vital extra minute of air if things go wrong.
All Apache pilots will carry a STASS fitted to their jackets when flying, so it is essential that using them is second nature.
Senior Instructor Gill continues, that once underwater "between five to eight seconds should be your ideal time to get yourself out".
"The reason we inform students to hold themselves back is, if you can imagine in the real world, once that aircraft is going in, there's going to be a lot of carnage going on, rotor blades etc.
"It's about holding yourself back, waiting for that to finish, then to make your escape."
The pilots do six dunks altogether, including in the dark and upside down.
Dunker instructor Dale Glen says it is vital that drills are correctly followed: "As soon as you're flipped upside down, your head plays funny tricks on you, so disorientation kicks in.
"We're very specific in our drills, in that orientation points, points of contact have to be there, pointing your eyes in the right direction so that your body follows."
He continues: "If you don't do that drill correctly – if you lose points of contact – it's very easy for your mind to start playing tricks on you when you're upside down and you can start going in the wrong direction.
"That's when people start getting into a bit of trouble."
For Captain Tim Helliwel, 656 Squardon 4 Reg Army Air Corps, the training is vital, although not necessarily something you look forward to.
He said: "It’s always a little bit stressful doing the underwater escape trainer, particularly the last runs where you are upside down and using the STASS bottle.
"It's the kind of training that you probably wouldn't want to go and fly over water, with the possibility of ditching, without."