HUNDREDS of metres beneath the surface of the ocean, sometimes all that Able Seaman Daniel Ponsford can hear is the ''clicking'' of the shrimps.
Other times, all he gets through his headphones are the raucous squeals of pods of excitable dolphins that are attracted to his boat, one of the much-maligned Collins-class submarines, HMAS Dechaineux.
''They make these really high-pitched squeaks and screams, actually they get pretty annoying after about six hours,'' he says.
Able Seaman Ponsford, a 21-year-old from Rockingham in Western Australia, is just one of a team of acoustic warfare analysts, or sonar operators, aboard the Dechaineux.
He told The Age his stories during a day on the Dechaineux, one of Australia's six Collins-class subs.
''You hear whales and dolphins and shrimp - their click-clicking through the water - you get to know all the sounds and they become second nature to you,'' he says.
A sonar operator quickly learns to differentiate the ''biologics'' - sealife - from ''contacts'' or other vessels.
They can also recognise the sound of rain on the water's surface and even the ''sound'' of the shore.
''Land mass actually sounds unique … dolphins, whale and shrimp, they become the largest sounds in the ocean, and you generally hear them in a mixed frenzy, and biologics actually enjoy hanging around vessels,'' AS Ponsford says.
''So you hear a bunch of biologics, and then you'll get a contact suddenly emerge out of them, because all that bio is louder than the ship. Then some days you'll just hear nothing, just an eerie silence.''
Unfortunately, the Collins class subs are better known for reasons other than an association with sea life. They are infamous for their problems - including early engineering and design faults as well as continuing maintenance issues. And reports say they continue to suffer from a series of ''designed-in'' problems.
While most of the problems that originally caused all the negative publicity - a failing combat system, excessive noise, engine breakdowns - have been remedied, problems still exist.
But on this trip the captain, Commander Jason ''J. J.'' Cupples, quickly demonstrates that the 3000-plus-tonne boat is capable and surprisingly agile.
Only minutes after it submerges, he takes it through a series of manoeuvres he calls ''the angles and dangles''.
First the ship noses down and moves swiftly towards the sea floor, then he steers it sharply to starboard, rolling on its axis.
Then he pulls it towards the surface and powers up, leaving him and his crew tilted forward at a strange-looking angle.
Next is a torpedo-firing exercise, using a group of hapless fishing trawlers floating above as prey. Air is sucked from the interior of the submarine, providing a palpable feeling of constriction, and water is fired from one of the torpedo shafts in a simulated attack.
Afterwards the sub breaches the waves and begins a cautious return voyage to HMAS Creswell.
After the demonstration of its capabilities, it begins a week-long journey around Australia's south coast to return to the Garden Island naval base in Western Australia.
The boat will remain under the waves - ''deep'', as they call it - for the entire week.
The return to the surface can often be confronting, according to Leading Seaman Andrew Edwards.
''You come back up to the surface and you turn your phone back on and you get your message bank, it's always interesting.
''[Either] you've won Lotto or your wife's left you.''