ABOARD THE USS SANTA FE — Riding 60 feet under the ocean surface in a 6,900-ton nuclear submarine that's longer than a football field, there is almost no sense of movement and very little noise.

The only sound is an occasional chirp over a speaker, signaling nearby marine life. The quiet is broken with a series of orders spoken in rapid-fire fashion:

"Chief of the watch, man battle stations!"

An alarm sounds.

"There is a hostile orange destroyer." "Orange" signifies potential enemy.

"Make tubes 2 and 4 ready in all respects."

More than a dozen crew members on the Los Angeles-class submarine are in the cramped 16-by-20-foot control room, a space dominated by twin periscopes, consoles, floor-to-ceiling electronics and airplane-like yokes to steer the vessel.

Eight more sailors are jammed into an adjacent sonar room.

The Santa Fe's captain, Cmdr. David Adams, a 25-year veteran of the Navy who spent time in Afghanistan in charge of a provincial reconstruction team several years ago, runs one of the periscopes up and down several times in quick succession to observe the suspect vessel.

After a series of directions are given in staccato brevity, Adams gives the order: "Shoot tube 2."

With that, there is a slight thud and whoosh as water is jetted out of the torpedo tube.

In actual combat, a MK-48 torpedo would have been fired, carrying 665 pounds of high explosives — enough to break the back of a destroyer or cruiser.

The training by the Santa Fe 12 miles south of Pearl Harbor is an aspect of the Navy the public rarely sees, but the "silent service" has continued to be a dominant force in Pearl Harbor — and in the Pacific — since World War II.

The 17 nuclear attack submarines based here — soon to be 18 — are the Navy's greatest concentration anywhere, surpassing the number of more familiar surface ships in Pearl Harbor by six.

At the same time, the overall number of U.S. submarines in service continues to fall, raising concern by some that the U.S. is losing a submarine arms race in the Pacific to the Chinese, and creating instability in the process.

A total of 224 Navy personnel are on the U.S. Pacific Fleet submarine force staff, there are 2,716 submariners assigned to squadrons here, and each of the 17 subs contributes $17 million annually to the local economy, officials said.

STRATEGIC BASE

The subs are here because Hawai'i's location 2,400 miles out in the Pacific gives them a head start on missions to the western Pacific to train with allies, and to keep an eye on ships and subs in a region that includes the world's six largest armies — those of China, the U.S., India, North Korea, Russia and South Korea.

The region also includes the shipping lanes of the Malacca Strait joining the Pacific and Indian oceans and through which passes one-quarter of the world's traded goods.

"The majority of our work is done forward-deployed," said Capt. Lindsay Hankins, chief of staff for the Pacific Fleet submarine force. "When you are trying to operate ships, there is a cost in doing that. Having them closer to the area in which you are operating them — in other words the western Pacific — makes it easier for us to get them there."

The Pentagon was concerned enough about changing dynamics in the region that in its 2006 Quadrennial Defense Review, it upset the 50/50 balance of attack submarines in the Atlantic and Pacific and ordered that 60 percent be based off the West Coast.

The Navy now has 52 attack submarines, with 30 of those in the Pacific. Most are Los Angeles-class boats, a class first deployed in 1976.

Five of the Navy's subs are from the new Virginia class, including the Hawaii and Texas at Pearl Harbor. A third Virginia-class sub, the North Carolina, is due in Hawai'i this summer.

In the Pacific, the Navy has subs in Hawai'i, Guam, San Diego and Bangor, Wash.

Attack submarines seek out other subs and ships, and can also launch Tomahawk cruise missiles, while Ohio-class submarines, or "boomers," carry 24 nuclear ballistic missiles.

Total U.S. sub numbers have dropped over the years, from 141 in 1971, to 115 in 1992 and 66 today.

The Heritage Foundation, a conservative think tank, last week said that unless the U.S. reverses the decline of its submarine fleet, U.S. military superiority in the Pacific "will continue to wane, severely limiting the Navy's ability to operate in the region."

DEMAND SURGING

Adams, the Santa Fe's commander, said the demand by combatant commanders in the Pacific and Middle East for submarines has increased two- or threefold over the past 10 years.

Norman Polmar, a naval analyst and author, said submarines mostly conduct surveillance. A Los Angeles-class submarine periscope has 18x magnification, and can watch airport or shipping activities.

Some subs practice by watching air operations at the Marine Corps base at Käne'ohe Bay.

"They are important," Polmar said. "They are important because they can go into areas where either for political or military reasons, we can't send a surface ship."

But he also thinks the U.S. has as many subs as it needs, and there are other areas the nation should invest more money in — such as amphibious ships to carry jump jets and Marines.

Multiple Pearl Harbor submarines enter and leave Pearl Harbor each month for training or six-month deployments on missions that are classified.

The subs have impressive capabilities. During recent training, when the Santa Fe was running on the surface at 15 knots (17 mph) and Adams spotted two humpback whales ahead, he gave an order and the 360-foot sub came to a stop within 100 yards.

Sonar operators can pick out dolphins swimming in front of the bow. Ships can be identified by the sound of diesel engines and propellers.

"We can pick up the Star of Honolulu because it's the only thing out there with four screws," said sonar technician J.R. O'Donnell, 32, of Danville, Ill.

The Santa Fe, commissioned in 1994, also practiced "angles and dangles" — ascending and descending at steep angles. The sub dove from about 200 feet to 600 feet and back multiple times in about a minute and half, at 25 degrees and 20 knots.

"We're going pretty slow. That's nothing," said Fire Control Technician Seaman James Collier, 21. The Navy only will say the subs are capable of diving to 800 feet and can reach 25 knots. The actual capabilities are much greater.

CLOSE QUARTERS

Life aboard for the crew of up to 143 means close quarters — in narrow passages, at meals and with berthing. There are five "heads," or restrooms, six showers and one washer and dryer aboard.

The crew sometimes has to "hot rack," meaning three sailors rotating in shifts for every two bunks.

Culinary Specialist 1st Class Salvador Rico, 37, was supervising lunch of steak and cheese sandwiches with grilled onions and grilled chicken in a 12-by-10-foot galley, or kitchen. He feeds about 120 people at every meal.

"The sailors, being the customers, they let you know right away — the food is good, or the food is bad," said Rico, of San Antonio.

Attack subs can go 60 to 90 days at sea before they need to be resupplied with food, officials said. That means little sunlight. Inside lights are brightened or darkened to mimic what's going on outside.

At sea, crew members wear blue coveralls known as "poopy suits" and their own personal choice in gym shoes.

All submariners have to volunteer for the duty and get a psychological evaluation for fitness. But they are paid more than surface sailors, Navy officials said.

"It's something that you kind of get used to," said Collier, of Chillicothe, Ohio. "You feel cramped up at your house, you can just start walking. Here, you don't have that option."

There are a few pieces of exercise equipment and some weights, but in general, you have to get along.

"The only thing that keeps you sane are your friends," Collier said. "As close as we are physically to each other, we become a close-knit family. I'd say more than anywhere in the military, this is more like a family."