America's most advanced nuclear submarine was slicing through the water off Hawaii last month, 400 feet under the surface, when a sonar operator suddenly detected an ominous noise on his headphones.
It was a faint thump … thump … thump — the distinctive sound of a spinning, seven-bladed propeller on a Chinese attack submarine called a Shang by the Pentagon and its allies.
A neon green stripe on his sonar screen indicated that the Shang was only a few thousand yards off the U.S. sub's bow.
"Sonar contact!" he yelled to 15 officers and crew in the dimly lighted control room. "All stations, analyze!"
Within seconds, the 377-foot-long Mississippi banked right and gunned its nuclear-powered propulsion system for one of the Navy's most difficult maneuvers: sneaking up behind another submarine and shadowing it without being detected.
Fortunately, the Mississippi was chasing a phantom, not a real Chinese sub. A digital recording of a Shang's audio signature had been piped through the U.S. sub's sonar system for a training exercise.
But the battle drill seemed urgently real: The mock Shang's course and speed were automatically fed into the Mississippi's targeting computers, the first step to launching one of its 27 torpedoes, something no U.S. sub has done against an adversary since World War II.
This is the largely unseen effect of the Obama administration's decision to send its newest vessels and warplanes to Asia over the last four years, a strategic rebalance intended in part to reassure Asian allies nervous about China's growing clout.
It has increased cat-and-mouse jockeying between the two largest navies in the Pacific, especially their growing submarine fleets. They track each other and train to fight with the same intensity of U.S. subs that once prepared to battle the Soviet Union.
When President Obama meets with Chinese President Xi Jinping at the White House on Friday, they are certain to discuss the growing military rivalry, especially in the South China Sea, where Beijing's buildup of disputed reefs has raised regional tension and sparked direct friction between U.S. and Chinese forces.
The Navy recently allowed a Times reporter aboard the black-hulled Mississippi, one of its newest and quietest fast-attack subs, for seven days, providing a rare look at the secretive world of the so-called Silent Service.
Beneath the waves, Cmdr. Eric Rozek and about 130 other officers and crew members, all men, did a series of complex training drills, often against an imaginary Chinese foe.
"This is our bread and butter," Rozek said as his sub tracked the mock Shang. "Because if we can do this, we can shift to using our weapons."
The exercise was meant to help the Mississippi, one of 12 Virginia-class fast-attack submarines, prepare for its first operational mission early next year, a six-month patrol in the Western Pacific that probably will include stalking actual Chinese subs and surface warships.
If war between the U.S. and China suddenly seemed probable, the Pentagon would send the Mississippi or its sister subs near the Chinese mainland, according to analysts and Navy officers familiar with Pentagon war plans.
They could launch cruise missiles at antiship missile batteries along the coast and try to torpedo Chinese warships before they could attack U.S. aircraft carrier battle groups.
China's antisubmarine systems have improved as part of a military modernization effort, and its growing submarine fleet "poses a significant and increasing threat," according to a new study of U.S. and Chinese military capabilities by Rand Corp., a think tank based in Santa Monica.
But U.S. subs "would likely inflict terrible punishment" if China tried to invade Taiwan, a small island that is a U.S. ally, or launch another major maritime operation, the 389-page study concludes.
The Mississippi was shifted to Pearl Harbor from Groton, Conn., in November as part of the rebalance. The Navy now has 43 of its 71 submarines in the Pacific.
About 20 attack subs, which carry only nonnuclear weapons, now are based at Pearl Harbor, making it the Navy's largest sub base. Four more attack subs operate from Guam, closer to the South China Sea.
The others are based in San Diego and Washington state. They include eight Ohio-class subs, the 560-foot-long "boomers" that carry nuclear-armed Trident II ballistic missiles and hide in the deep ocean in case of nuclear war.
China, in turn, operates at least 62 diesel- and nuclear-powered subs. It could boost that to 78 over the next five years, according to a Pentagon report released in May.
They include four Jin subs that are believed to carry medium-range, nuclear-armed ballistic missiles. China "will likely conduct its first [submarine] nuclear deterrence patrol sometime in 2015," the Pentagon report states.
China has expanded conventional sub patrols into the Indian Ocean and the Arabian Sea as far west as the Horn of Africa. Keeping track of them has added to the Navy's workload, Adm. Harry B. Harris Jr., top U.S. commander in the Pacific, said Sept. 17 at a Senate Armed Services Committee hearing.
To help counter the threat, the Navy deploys special spy ships and P-8 surveillance aircraft, which drop sensitive sonar buoys and underwater listening devices. But it also relies on its attack subs in the Pacific.
Off Hawaii, about a dozen of the Mississippi's officers and senior petty officers assembled one recent afternoon in the mess, where the crew eats, to discuss the day's battle drill.
For the purposes of the exercise, war loomed in the Pacific. A nuclear-armed Jin submarine — not a Shang this time — was lurking off an imaginary U.S.-allied nation resembling Japan. The Jin was from "Churia," not China.
The Mississippi had 36 hours to find and, if necessary, destroy the enemy sub, Lt. Cmdr. Dennis Milsom, the executive officer, explained. The officers listened intently, clustered near an ice-cream machine. Most were serving their first or second sub tours.
Among them was Lt. Ray Wiggin, the sub's weapons officer. His crew handles the Mississippi's torpedoes and dozen Tomahawk cruise missiles. He was told to prepare to launch a salvo of cruise missiles at Churian targets on shore.
Nerves were on edge. War had not broken out. And the mock orders permitted an attack only if the Jin was clearly hostile.
"If they are [attacking] a freighter, we have authority to engage," said Milsom, a ginger-haired graduate of Penn State.
Otherwise, they should track the Jin and await a "strike tasking," an order from the Navy's 7th Fleet commander confirming that war with Churia has begun.
"We're pretty binary," Milsom said. "We're either going to sink them or we're not."
On this day, they did not. The exercise ended without an exchange of fire.
It is not an easy life. Even when the boat surfaces or pulls into port, only the few crew members on the bridge, the tiny observation area atop the conning tower, see the sun.
They also face relentless pressure to get "qualified," proving they are proficient in an assigned task.
"You don't get a lot of free time," said Chief Petty Officer Heath Hooper, an electronics mate who helps run the 40,000-kilowatt nuclear reactor that supplies power. "It's always, 'Why are you reading that book? Why aren't you qualified?'"
In the torpedo room, Chief Petty Officer John Stitt drilled his men on loading Mark 48 torpedoes. An experienced crew can load a tube in about nine minutes, but his crew wasn't fast enough yet, Stitt said. So they drill continually.
The torpedoes, each 19 feet long and weighing nearly 2 tons, are stacked in racks in the low compartment. Some carry high-explosive warheads and others are duds used for training.
"Weapon in motion," yells Torpedoman's Mate Cody Krollpfeiffer as a hydraulic lift readies a Mark 48 to be jammed into one of the four tubes.
Gone are the days of "up periscope" on Virginia-class subs, the command to raise an optical periscope so the captain can survey the surface and launch an attack.
Instead, the Mississippi raised its photonics mast, which uses a high-resolution digital video camera, above the waves. Other masts can scan the surface with radar or vacuum up digital communications traffic and other intelligence.
Still, the technology has its limits.
In one exercise, officers practiced raising the mast in a crowded shipping lane such as those in the South China Sea. A video screen that normally shows the ocean instead displayed computer-generated silhouettes of a trawler, a cruise ship and two Chinese warships — a Shang sub and a guided-missile destroyer known as a Luhu.
But initial attempts by Wiggin, the officer in charge, didn't please Rozek, the skipper. To avoid detection, Wiggin raised the mast and quickly surveyed the Chinese ships, then lowered it. Then he did it again. And again.
"We're ducking and running, ducking and running, ducking and running," Rozek said.
Wiggin should have moved the sub to where it could keep the mast up safely "because pretty soon the Shang is going to submerge and become invisible," Rozek said.
And though the sub's sophisticated sonar sensors can pick up tiny clicks from schools of swimming shrimp, the so-called passive sonar doesn't always work against China's most advanced subs, which can operate in virtual silence. Sometimes they disclose their position by banging a hatch or making another inadvertent noise.
The solution is to use active sonar, a ping of sound that echoes off another vessel. That reveals the enemy but also can give away the ship using sonar.
"If we're not putting artificial noise into the water, we don't generally find them," said a sonar operator aboard the Mississippi.