NHA TRANG, Vietnam -- For much of Staff Sgt. Humberto Santiago's life, the sea has been his second home.Growing up in Puerto Rico, the Army diver was drawn to the water at an early age. When he was 11, he became a certified scuba diver -- a year before he was technically allowed to, but his father convinced the instructor to teach him. He could also be seen spearfishing or behind the wheel of a boat.Now a squad leader with the 7th Engineer Dive Detachment in Hawaii, he estimates he has spent roughly three weeks of "bottom time" while submerged in support of Army missions over the past 13 years.One time, during a training exercise, he even held his breath for 4 minutes and 20 seconds under the water."The ocean is a peaceful place," the 40-year-old recently said. "That's where I feel the most at home. It's not a place for everyone, but it's a place for me."DIVING INSTRUCTORSantiago is part of the Army's small, elite diving force. There are roughly 150 divers in the entire service, and while mainly based out of Hawaii and Virginia, they also operate around the world.It's such a unique community that Army divers often work with those who they began their careers with, even their Army instructors at the Naval Diving and Salvage Training Center in Panama City, Florida. The school offers joint programs to all military divers."It's very close-knit," he said.At 130 days long, the entry-level engineer diver course is extremely difficult, with the majority of students not passing it. The course mentally and physically drains students as they learn specific tasks, such as underwater demolition, engineer reconnaissance, and mine and counter-mine operations in an intense environment.When he went through, Santiago said he was the only one to graduate out of his class of 10 students.While grueling, he said, the course prepared him for the rigors of the job that at times has divers working blindly in dark, murky water at more than 100 feet below the surface.After he became a more experienced diver, he was given the chance to return to the school as an instructor."It felt like coming full circle," he said. "It's something that I always wanted to do. I always want to come back."He still remembered how hard it was as a student back then, but he refused to take it easy on the classes of prospective Army divers."Our job is innately dangerous. It is not an environment that we are adept to survive in," he said. "We have to be hard on them so they understand the risks. Every time they leave [the] surface, they could potentially die."The strict demeanor of Santiago as an instructor initially left a sour impression on Spc. Timothy Sparks, a diver who now works for the sergeant.Sparks, 22, of Jefferson, Georgia, described him as a "very meticulous" instructor who even zeroed in on him because he did not have his backpack straps tied up."I actually told him recently that I didn't like him at the time of dive school because he was always a pain to deal with," Sparks said, smiling. "He would always give you trouble for every little thing and I never understood it."At the 7th Dive Detachment, which is part of the 8th Theater Sustainment Command, Sparks said his former instructor is now his squad leader and a completely different person than back at the schoolhouse."He's there for me," he said. "I get to work with him on a day-to-day basis and he's phenomenal. He's very helpful and very wise in life and diving."Spc. Lamar Fidel is another team member whom Santiago continues to help mold into a better diver.In his entry course, Fidel thought he was also in Santiago's target sights since he often pestered him about quitting."I was thinking he was picking on me … but he was doing it for a reason," Fidel, 28, of Atlanta, said. "He was doing it because he wanted to see if this guy is going to be an asset to my team. If he's not, then I don't want him to be a diver."MASTER DIVERBack when he was a private, Santiago recalled being in his Soldiers' shoes and facing a similar leader, a former first sergeant at Fort Eustis, Virginia, who urged him to work harder."I took a page out of his book," Santiago said. "He was your typical Army first sergeant -- very stern. He was [also] extremely smart. He was a very good example to follow."One day, the first sergeant asked Santiago when he would begin prepping to be a master diver, the pinnacle of Army dive training. Santiago hesitated and replied he would start when he becomes a staff sergeant. The first sergeant quickly countered him with a "no" and told him to start now from day one."I always took that to heart," Santiago said.As a tribute to his efforts, Santiago received an email to attend the master diver course later this year. The decision is unique for an E-6 diver not on promotable status, like Santiago, to be chosen to try for master diver, a title typically reserved for E-7s and up."It's a very challenging course," he said. "It's probably the hardest thing that I will do as a diver."VIETNAM CONNECTIONAs part of his unique set of missions, Santiago recently served as a diving supervisor for his younger divers who dredged sediment from 80 feet below on the sea floor.It was Santiago's third underwater recovery mission and second to Vietnam in support of the Defense POW/MIA Accounting Agency. The other one was in Palau to recover the remains of American service members from World War II.As he supervised his divers, he could not help but reflect on the 20-something-year-olds tasked to do the mission, which was in search of six Soldiers who went missing after their Chinook helicopter crashed in southern Vietnam in the war."They weren't even born when these people died and they're here trying to find them," he said during the mission, which ended in mid-April. "It's a brotherhood. It doesn't matter what service you are, there's a promise and we're going to find you."His previous Vietnam mission was even more emotional for him. With it being his first time in the country, the mission gave Santiago a chance to reconnect with his father, who died in 2003."It brought me back to where my dad served," he said, adding his father was an Air Force security forces patrolman during the war. "It was an opportunity I couldn't pass up."When asked what his father -- who did not get the chance to see him in uniform -- would think of his son now, Santiago simply replied, "He's proud of me."The mission, which was in search of two Navy pilots in an F-4 Phantom fighter jet that crashed in the Gulf of Tonkin, recovered a bone later identified to be from one of the pilots."That was a win for all the hard days that you don't find anything," Santiago said.Those hard days are often compounded by the weather, which he describes as their biggest challenge."We have all the equipment, we have the training, we have the personnel, but nobody can control Mother Nature," he said. "You're at the mercy of the ocean, which is one of the most powerful forces on Earth."NEAR-DEATH EXPERIENCESantiago, though, keeps being drawn into those sometimes unforgiving waters.When not working, he has competed in freediving events, where a diver cannot use scuba gear while diving at deep depths.In one of those competitions in July 2007, he suffered a shallow water blackout, which is basically an underwater "faint" due to a lack of oxygen to the brain.Fortunately for Santiago, a friend of his noticed him passed out in the water and pulled him up to the surface. "If it wasn't for him, I wouldn't be here today," he said. "But it hasn't stopped me, because it's something that I love to do. I'm not reckless by any means, but I will do what I love."From being a young kid swimming off the coast of Puerto Rico to his adult years as an Army diver, the sea has been an essential part of his life.As soon as he gets into the ocean, a certain energy from the water envelops him."I call it salt-water therapy," he said. "You just jump in and you feel invigorated."And if he has his way, he will one day remain in the ocean forever. He has already purchased an "eternal reef" from a company that promises to put his cremated ashes in an underwater urn, along with his name on a plaque."I'm going out there," he said with certainty. "That's where I'm staying."