Sidharth Kaushal, a naval expert at London's Royal United Services Institute, describes the USS Ohio and its sister boats, the USS Michigan, USS Florida and USS Georgia, as one-stop shops for getting missiles and troops in close to an adversary's territory.

And that could be significant when compared to adversaries like China, which maintains a robust anti-ship missile capability but whose defenses against submarines are still being upgraded and refined.

'A lot of firepower very rapidly'

Though it no longer carries nuclear missiles, the USS Ohio is nuclear powered, as are all US Navy submarines. Known as a nuclear-powered guided-missile submarine (SSGN), the Ohio is driven by a nuclear reactor providing steam for two turbines, which turn the sub's propeller.

The Navy calls its range "unlimited," with its ability to stay submerged constrained only by the need to replenish food supplies for its crew.

The submarine's comparatively large size and power allow it to carry 154 Tomahawk cruise missiles, 50% more than US guided-missile destroyers pack and almost four times what the US Navy's newest attack subs are armed with.

Each Tomahawk can carry up to a 1,000-pound high-explosive warhead.

"SSGNs can deliver a lot of firepower very rapidly," said Carl Schuster, a former Navy captain and director of operations at the US Pacific Command's Joint Intelligence Center.

"One-hundred and fifty-four Tomahawks accurately deliver a lot of punch. No opponent of the US can ignore the threat."

While the Navy could amass a larger number of destroyers to deliver missiles in even greater numbers, as a standalone, hard-to-detect unit, the Ohio-class guided missile submarine is in an ocean by itself in America's arsenal, said Bradley Martin, a former Navy captain turned naval researcher at the RAND Corp think tank.

"The SSGN remains the platform with the single largest ability to deliver conventional missile payloads," Martin said.

The magnitude of that firepower was shown in March 2011, when the USS Florida fired almost 100 Tomahawks against targets in Libya during Operation Odyssey Dawn. The attack marked the first time the SSGNs were used in combat.

Libya is not China, however, and the People's Liberation Army Navy (PLAN) has numerous and improving anti-submarine warfare capabilities that Libya does not.

Beijing has been investing substantial resources in a growing fleet of submarine hunting aircraft and frigates and dozens of hunter-killer submarines, all with the purpose of sinking enemy submarines.

But for all its advancements, China is still playing catch up. It wasn't a Cold War submarine power -- and in sub hunting, numbers need to be complemented with experience.

"The question of whether they can do so is a function of how well networked these assets are, and how well trained the operators are -- with analysts' opinions on the PLAN's progress in this area differing quite a bit," naval expert Kaushal said.

If the USS Ohio is operating out in the Pacific, finding it becomes harder, as China's anti-submarine force was designed to work closer to its shores, he said.

But even closer to shore, the Ohio has a stealthy advantage, the analysts say. It's quieter than other attack subs in the US fleet and still would present a challenge for China to find in waters closer to its shores.

That means it can bring its dozens of land-attack missiles closer to targets well inland, analysts said.

"SSGNs can get into forward positions by virtue of their stealth and strike targets deep within defended hostile areas," Kaushal said.

And they have the element of surprise. Early warning air defenses that search for planes or surface ships moving toward an adversary lose their effectiveness when a sub pops up close to a coastline.

"SSGNs provide the US Navy with a greater long-range strike capability than almost any other asset in its service," said Kaushal.

Adapting to new threats

The Ohio is one of the quietest submarines ever built.

Conceived in the 1970s and commissioned in 1981 as the first of the US' Trident nuclear missile submarines, it was the epitome of strategic deterrence.

The Ohio and the 17 identical submarines in its class all carried 24 Trident intercontinental ballistic missiles, each with as many as eight independently targeted nuclear warheads. In theory, one sub could have wiped out multiple cities in the Soviet Union in a single launch.

The subs were designed to stay submerged for months at a time with the mission to rise and deliver a devastating counterattack to any Soviet nuclear missile strike on US territory.

Under the sea, they went deep and stayed quiet, making it difficult for the Soviets to keep tabs on them, thereby preserving their deterrent value.

But when the Cold War ended and US tensions with Moscow eased, the need for so many ballistic missile submarines decreased. The Ohio, Michigan, Florida and Georgia were going to be decommissioned, but then the Navy decided their stealthy abilities could be reallocated to help answer evolving threats.

"It's going to be a key contributor to the Navy's participation in the global war on terror," Capt. David Norris, the Navy's SSGN program manager said in 2007.

That meant making room for special forces.

Spaces for the nuclear missiles were changed into new berths and bathrooms, enough to accommodate 66 Navy SEALs or other special operations troops.

Missile tubes were turned into lockout areas that could be flooded and drained for divers to exit and enter the sub. Deployments can also be made with a mini-submarine.

Warm water showers were added for those troops to bring up their body temperatures once back aboard. Drying rooms were built for their equipment.

There's even room for a simulated firing range.

Meanwhile, the sub has the space and equipment to be a command and control center, able to direct operations on nearby coastlines while maintaining a covert position under the ocean.

The Ohio was the first of the four ballistic missile submarines to be converted to a guided-missile sub, going on its first mission as such in 2007.

Since then, the Ohio has operated out of Naval Base Kitsap in the state of Washington and is often forward deployed to the US sub base on the Pacific Island of Guam.

Tensions in the Pacific

Last week, the submarine and its crew of more than 150 staged exercises with reconnaissance elements of the US Marine Corps Third Expeditionary Force off Okinawa, part of what's known as the first island chain in reference to hurdles China must pass through to get its military forces access to the open Pacific.

Photos supplied by the US Navy's 7th Fleet showed the submarine working with Marines in small boats and Osprey tilt-rotor aircraft.

"Every time we train with our Marine Corps counterparts, it sharpens our ability to respond fluidly to regional challenges, deliver combat-tested capabilities and prevail in day-to-day competition, in crisis and in conflict," Capt. Kurt Balagna, commanding officer of the Ohio, said in a news release.

"This unconventional concept could be a viable option in our warfighting toolkit," the Navy captain said.

Maj. Daniel Romans, commander of the Okinawa-based Force Reconnaissance Marines, said the exercise was important to his troops' versatility.

"As the stand-in force in the first island chain, it is critical that Force Reconnaissance Marines are capable of being employed across a myriad of US Navy platforms in order to enhance the lethality of the fleet," Romans said.

Kaushal, the RUSI analyst, said the Ohio -- or its Pacific-based sister boat the USS Michigan -- could be vital in stopping any possible US conflict with China before it starts.

"To use a hypothetical example, if intelligence suggested a short bloodless land grab against an uninhabited disputed island was being envisioned by China, the Ohios could be used to disembark forces sufficient to hold off a limited Chinese force if the Ohios were the closest assets to the scene.

"Getting US forces to the island before the Chinese got there would be key to preventing a bloodless fait accompli and forcing the Chinese to choose between escalating and backing off," Kaushal said.

If the Okinawa exercise was being used to send a message to China, it wouldn't be the first time the SSGNs would be in that role.