TENSION hangs over a four-storey block in Sembawang Camp in the wee hours of a Monday morning, as 74 trainees of the 40th Combat Diver Course toss fitfully in their cabin beds.
At precisely 2.30am, Second Warrant Officer (2WO) S. Rajendren strikes a handheld grenade simulator known as a thunderflash and waves it into a shallow drain.
The blinding explosion galvanises other trainers, already in position. They pound on the wooden bunk doors, screaming unabatedly while tossing unguarded rucksacks and loose footwear down the corridors, sending befuddled trainees scrambling after them.
Drugged with sleep and sagging under the weight of their olive-green packs, the young men crawl down the stairs on all fours, spilling onto the concrete parade square like a colony of evicted egg-carrying ants, to a drenching welcome with fire hoses.
On cue, several trip flares burst into life, illuminating the chaos in a bright orange glow, casting grotesque caricatures of the disoriented men onto the walls of the building - a shadow play gone awry.
From a corner, a machine gun punctuates the cacophony with a shattering staccato that reverberates in the cool night air.
Standing on a platform above the maelstrom of swirling smoke and glistening rain in his dark blue instructor's T-shirt and camouflage slacks, 2WO Rajendren is the man in charge of this mayhem.
Feared and respected in equal measure by the trainees, the training officer's drills are their first taste of the long night ahead. "On your belly, on your feet, on your backs! Push-ups, crunches, jumping jacks in position, ready!"
The bewildered trainees are out of sync, unable to keep up with the impossibly quick commands belted through the megaphone slung from his shoulder.
Hawk-eyed instructors prowl the ranks, blasting banshee-like whistles and stabbing torch beams into the sodden jumble of limbs.
Twenty minutes into the commotion, every trainee is soaked and utterly miserable.
Although every one of them knows this moment must arrive, nothing can prepare them for the actual bedlam of "Breakout" - the opening act of the combat divers' Team Building Week.
Held during the seventh week of the course, this naval diver's rite of passage is better known as Hell Week.
Five months earlier, 450 young men began this journey when they received posting orders to the Naval Underwater Medical Centre for a vocational assessment.
A battery of tests would ensure that only the finest physical specimens were selected.
Within pristine confines, orderlies measured each man's body fat and lung capacity, probed the integrity of his ear drums, wired him to an electrocardiograph to monitor his heartbeat, and then packed him into a pressure chamber sealed with double hatches to see if he could bear a simulated dive to a depth of 10m.
Still intact, each candidate then had to change into physical training attire to attempt his best scores in the standing broad jump and chin-up stations, strip to his trunks for a 25m swim, then present himself for a face-to-face interview with a senior instructor, usually a warrant officer with years of diving experience.
Here, any fears of the deep a candidate might have were put to rest, while his character and motivation were probed with questions such as: "Are you afraid of water?", "What school co-curricular activities did you take part in?" and "Do you want to be a naval diver?"
The final stop of the day was a psychological assessment. The multiple-choice test involved a multitude of questions to determine if he would be able to cope with the stresses divers experience frequently on the job.
Those who cleared the selection moved on to nine weeks of Basic Military Training (BMT) modified to include pool training and an introduction to maritime culture with terms such as "port" and "starboard" replacing "left" and "right".
Now, the numbers were whittled down again, as those who failed the water confidence and physical fitness standards were posted to other units.
Of the 450 initially called up, less than a quarter entered the five-month Combat Diver Course to learn military scuba diving and push their bodies to unprecedented levels of physical endurance.
Private Nigel Tan, a member of the Ngee Ann Polytechnic Scuba Diving Club before national service, felt both privileged and nervous when he received his posting letter.
"As a leisure diver, I followed a divemaster around and looked at fish," says the 21-year-old electronic and computer engineering graduate. "Here, we have to carry heavy equipment, deal with low visibility and still complete a mission in a given timeframe."
At the start of BMT, he could manage just two chin-ups; at the end, he was doing more than 20. "My body has toned up a lot," he says.
With twin scuba tanks tipping the scales at 38kg and a rubber boat weighing twice as much, divers have plenty of heavy lifting to do. Weapons and special gear for each mission add to the load each man must carry.
To keep themselves in top shape, divers run, swim and work out up to three times a day and, unlike most other soldiers, are allowed to wear their physical training attire at all hours.
"It is the nature of the job," explains 2WO Rajendren.
"A simple thing such as swimming underwater against a current takes a lot of strength; lifting objects and diving with equipment needs plenty of fitness. You need to be strong on land in order to be strong in water."
Alongside the commandos, divers are among the fittest men in the armed forces, with nearly every one of them having earned a gold award in the SAF Individual Physical Proficiency Test (IPPT).
The IPPT holds commandos and divers to a higher standard than other soldiers, requiring full marks (5/5) in every station and a 2.4km run in less than 9min 15sec for a serviceman under the age of 25 to attain a gold award.
Other servicemen need just 4/5 points for each station and a run time of under 9min 45sec.
The result of so much exercise is that trainees end up supremely fit by the end of the Combat Diver Course, boasting perfect pectorals and a clearly defined six pack.
"The PT sessions were my favourite part of the course," says Private Guo Jingyang, 21, who managed a count of 37 chin-ups at his best. "The instructors didn't just conduct the PT, they did it together with us, even if they were 20 or 30 years older than us."
All the physical training leads up to Hell Week. But unlike PT, where the focus is on building bodies, this week aims to train the mind by putting the trainees through extreme stress in order to identify the final list of combat divers.
"Team Building Week was definitely one of the toughest weeks I've been through," says Pte Guo, who emerged top graduate of the course. "The pain was almost unbearable at times."
Of those who started out in the 40th Combat Divers Course, only 74 are left at the start of Hell Week - the rest have dropped out along the way.
Three eight-hour shifts of instructors are needed to run the show, dishing out an intense and relentless slew of exercises called "evolutions", each calibrated to take trainees to the brink of exhaustion.
Wet and Sandy; Cold Treatment; Around the World - the men know them all by name, but each evolution varies in the way it is conducted to keep them on their toes.
The most dreaded involves a black rubber dinghy called the Inflatable Boat - Small. Weighing 76kg when empty, the boats are made heavier when instructors throw in a large tyre or a few sandbags for good measure.
Overhead boat push-ups and boat tosses - where a crew of eight men throw the boat into the air and catch it on the way down - are feats that test the coordination of even the best teams.
To hoist one of these boats overhead takes careful choreography - a slip could result in the boat crashing to the ground, and punishment for the crew.
One spectacle that heralds every meal time is the Chow Run.
This is when the Hell Week cohort can be heard thundering around the parade square holding their boats overhead, earning their right to eat by running and belting out military songs for an average distance of 2km.
"Being under that boat is really painful," recalls Private Marcus Nathan, 24, a former firefighter who was drawn to the challenge of becoming a regular combat diver during a navy exhibition.
"Your hands get tired, your shoulders become weak, your neck starts to hurt, and your legs are burning - but you have to keep going."
Only when the instructors are satisfied with the run are the trainees lined up and hosed down for the sake of hygiene, then given 30 minutes to devour a plate of rice heaped with chicken, fish and vegetables, accompanied by a bowl of steaming soup and fruit.
Snacks, such as cake, doughnuts and chocolate bars, are added to the menu to give divers a total of 4,000 calories a day - 11/2 times the dietary allowance of ordinary SAF rations, but trainees are subjected to such intense physical activity during Team Building Week that most still lose weight at the end of the five days.
Through it all, they will sleep a total of just four hours.
"At first, I was too fired up to think about sleep," recounts MID Guo. "But by the fourth day, we started to fall asleep when walking, when paddling the boat, even in the pool."
This severe test of sleep deprivation prepares the men for real-world missions when divers may have to operate continuously for several days, says 2WO Rajendren. With their bedraggled green fatigues now brown with mud and dirt, the trainees stand at attention with their wooden boat paddles as the commander of the Naval Diving Unit, Colonel Tan Hong Teck, fixes them with a steely eye, his US Navy Seal badge sitting proudly above his right chest pocket.
"40th batch!" the beefy officer roars.
"Yes, sir!" the men thunder, their voices echoing around the parade square where fellow divers from all over the camp are lining the corridors of surrounding blocks to witness the sight.
"Are you tired?" he asks, knowing the answer to his own question.
"No, sir!" they thunder.
"Then go and get Wet and Sandy!" he bellows.
As one, the men sprint for the sea, plunge into the water, emerge and thrash about on the beach, knowing this might be the last time they have to endure this evolution.
Within minutes, they are once again lined up in front of their commander, dripping wet and encrusted in sand.
"40th batch!" he booms, raising a closed fist to quiet the desperately eager trainees. "I declare Hell Week secure."
Pandemonium erupts as trip flares and machine guns go off, marking the end of the gruelling ritual. The parade-perfect contingent is now a throbbing mess of men - cheering, crying and hugging each other in their steaming, stinking, salt-marinated uniforms.
A line gradually forms as the smoke clears; senior commanders and instructors move down to shake each trainee's hand in turn.
Heading back to the barracks, the trainees are applauded by passers-by. Even those driving by stop and roll down their windows to congratulate the new "HellWeekers".
Despite having cleared the toughest week of the course, some will still fall out for medical reasons or for failing to meet the increasingly high physical fitness standards.
For Private Gabriel Ong, 22, learning that he did not pass the course was devastating.
He was just one minute over the 27 minutes cut-off for the 6km run - one of five "vetoes" a trainee must pass in order to become a diver.
"When I was told that I could not continue the course while we were rehearsing for graduation, my heart sank," he recounts. "Everything that I had trained for was shattered in that instant."
There was no doubt that Pte Ong was fit - he attained a gold award for the IPPT - but that was not good enough. "I really wanted to be a diver," he says wistfully. "It is different from the usual army life; it is fun, and it has prestige."
For the remaining trainees, several more weeks of instruction in advanced diving techniques will follow Hell Week before the end of the Combat Diver Course, when mothers, fathers and girlfriends will pin the silver professional diver badge with a dark blue felt backing on the 69 men who made it.
But graduation is just the beginning of a frogman's entry into the underwater elite.
Some full-time national servicemen who clear the course will join the navy's Clearance Diving Group, where they will keep the waters around naval bases free from mines and other obstacles, perform maintenance work on ships such as the cleaning of propellers, and dispose of live munitions, usually war relics, found on the seabed.
Others will join the Underwater Demolitions Group, whose divers scout potential beach landing sites and remove underwater obstacles so friendly troops can go ashore.
A select few will be sent to Officer Cadet School and graduate nine months later as second lieutenants, ready to lead dive teams.
"In the Combat Diver Course, the biggest challenge for trainees is fulfilling all the vetoes in water competency," says 2WO Rajendren, who has trained numerous divers in his 20-year career. "My greatest satisfaction is building up someone from nothing to becoming a fish in the water."