Operating beneath the ocean's surface, a submarine's strategic value is often tied directly to its ability to navigate in or near enemy waters without being detected to conduct reconnaissance or attack missions.
For years, the United States has maintained a technological edge over the submarines developed by rival nations, but recent advances made by Russia and China have sparked concerns of an emerging threat to American undersea superiority.
The Navy's 70-boat submarine fleet is made up of three major types of boats: ballistic-missile submarines, attack submarines, and cruise-missile submarines.
The Navy currently fields 17 Virginia-class fast attack submarines, which are built to operate in the world's littoral and deep waters.
Virginia-class submarines can launch torpedoes at other submarines and at ships. They can also launch missiles at ground targets, gather intelligence, and deploy Navy SEAL units for special operations.
"Their inherent stealth, endurance, mobility and firepower directly enable them to support five of the six maritime strategy core capabilities -- sea control, power projection, forward presence, maritime security and deterrence," the Navy said.
Incorporating "acoustic superiority" that is expected to provide unparalleled stealth capability, the USS South Dakota will be used as a "demonstrator to prove out advanced technologies," according to Navy spokesperson Lt. Seth Clarke.
Lessons learned from South Dakota will be incorporated into later Virginia-class submarines -- "increasing our undersea domain advantage, ensuring our dominance through the midcentury and beyond," Clarke said.
"Stealth capability is one of the crucial advantages of submarines ... the Virginia-class brings capability and capacity that is so crucial as we head into potential peer conflict down the road." according to Randy Forbes, a former US representative who served as chairman of the Seapower and Projection Forces subcommittee of the House Armed Services Committee while in Congress.
The 360-foot USS South Dakota is scheduled to officially enter the fleet in August 2018.
The Navy plans to upgrade its Virginia-class boats while developing the next-generation, nuclear-armed Columbia-class.
One of those upgrades includes outfitting Virginia-class submarines with additional "Virginia Payload Modules" for Tomahawk or next-generation missiles to provide more strike capacity.
Most Virginia-class submarines currently feature two Virginia Payload Tubes, each capable of launching six Tomahawk cruise missiles, according to the Navy.
Expected to be operational by 2024 or 2025, that added firepower will help fill the gap as the Navy plans to retire several of its aging fast-attack and ballistic-missile submarines in coming years, according to Jerry Hendrix, a retired Navy captain and senior fellow at the Center for a New American Security.
Brewing submarine rivalry
Forbes said the United States has traditionally been able to track foreign submarines but recent advancements made by the Chinese and Russians highlight the need for developments like those seen on the USS South Dakota to maintain American undersea dominance.
Russia has invested heavily in developing its own underwater stealth capabilities in recent years and their submarine technology is approaching the level of the US fleet, much like the peer-to-peer comparison seen during the Cold War, according to one congressional aide familiar with the issue.
While the Russian military is not necessarily building a large quantity of submarines, it is developing boats with advanced quieting capabilities that are "very competitive" with those in the US fleet, said Hendrix.
"Russia is modernizing its existing fleet of Oscar-class multipurpose attack nuclear submarines and producing their next generation Severodvinsk Yasen-class," US Pacific Command chief Adm. Harry Harris told the House Armed Services Committee
earlier this year.
"The Russians are making a leverage investment ... with submarines you don't have to build a lot of them," according to Hendrix.
The United States has also recently noticed an uptick in Russian submarine activity
-- conducting "spurts" of heavy patrols across various regions, according to the aide and a US Navy official.
"They can't maintain the presence we do with 24/7 operations, but this activity is meant to push the boundary and to see what the US reaction will be," the aide said.
But while the United States continues to monitor foreign submarine activity, the increase in Russian patrols is widely viewed as presence and testing maneuvers that do not pose an immediate military threat.
It's something the United States should be reacting to but not overreacting to, the congressional aide told CNN.
In 2016, US Navy commanders told CNN that Russian submarines had become increasing assertive in the Atlantic Ocean and that their undersea activity was reaching levels unseen for decades.
The commander of the USS Missouri submarine, Fraser Hudson, assessed at the time that the renewed Russian activity is not just "a political statement but the Russians are seeking to gain experience in case hostilities were ever to break out between it and the United States.
"Honestly, I think it's operational experience. You maintain the experience in those (areas of responsibility) so that if anything were ever to happen, they have experience," he said last year.
A Russian sub also turned up off the coast of Florida in 2012, and the USS Missouri was called on to track it.
In addition to flexing their military muscle in an attempt to show "they still are or want to be a dominant super power," the Russians are also trying to test whether their submarines can travel for long periods without being detected, according to the aide.
"The Russians are probably at about 80% of the capacity they were at during the Cold War," according to Forbes, who also noted that they are constantly pushing the envelope in terms of capability.
China, on the other hand, is opting for a quantity over quality approach when it comes to building up its own submarine fleet.
"The Chinese are not as advanced but are getting there and they are producing diesel submarines in numbers," the congressional aide told CNN. "In the end, numbers are a capability in themselves."
Hendrix also noted that there is no indication the submarines being developed by China are close to being as advanced as those in the US arsenal but warned their approach of producing a large number of boats does pose a threat.
"Quantity can have a quality," Hendrix said.
In addition to adding to the size of its submarine fleet, Harris warned earlier this year that they are also advancing their undersea capability.
"China is improving the lethality and survivability of its attack submarines and building quieter, high-end diesel and nuclear-powered submarines," he said.
Both China and Russia have also increased their presence in the Indo-Asia-Pacific region, where Harris said 230 of the world's 400 foreign submarines are operating.
Roughly 160 of those 230 submarines belong to China, North Korea, and Russia, according to Harris.
Forbes said the United States must also develop a strategy to counter Chinese and Russian activity in "gray zones" where they are incrementally expanding their presence by strategically "fighting and competing" through military posturing.
China's claims in the South China Sea represent one glaring example as to how they've been able to successfully implement this type of strategy in a way that allows them to expand their military reach without engaging in direct confrontation, according to Forbes.
Forbes also explained that the United States has yet to develop a coherent strategy to counter Russian and Chinese gray zone activity -- a challenge that will require both the Navy and Congress to reconsider the way it utilizes American sea power.
"Major strategy is not some secret play you pull out in the fourth quarter of a football game," Forbes said, adding that if US military leaders don't articulate a strategy to Congress there is no way a plan will be implemented.
By most accounts, the United States still maintains the most capable submarine fleet in the world, but while the addition of the USS South Dakota represents a leap forward in technological advancement, some argue the Navy still faces challenges.
Despite President Donald Trump's request for additional defense spending, years of budget cuts and continuing resolutions have had a severe impact on the Navy's maintenance and shipbuilding efforts.
The Navy plans to build 30 Virginia-class submarines and replace its 14 aging Ohio-class boats with 12 Columbia-class submarines, the first of which is expected in fiscal year 2021.
In March 2016, the Government Accountability Office
reported that total program acquisition costs will be about $97 billion, including $12 billion for research and development and $85.1 billion for procurement.
There are also discussions about extending deployments and service lives for existing submarines to help make up the difference.
But even if the Navy is able to achieve its goal of producing two or three new fast-attack boats per year, the service will only have 41 attack submarines by 2029, according to the Navy's FY17 30-year plan
An attack submarine inventory of 41 boats falls well short of the 65 to 70 boats needed to effectively maintain undersea dominance, according to Hendrix.
Harris told the House Armed Services Committee earlier this year that he agrees with assessments indicating a need for 66 fast attack submarines.
The need to continuously upgrade submarine stealth capabilities also presents a challenge for the Navy, according to Forbes, who said there comes a time when the cost for another decibel of quietness becomes too expensive.
Increased costs and the development of new detection techniques that do not rely on the sound a submarine makes could change the way the US Navy thinks about undersea warfare, according to Forbes.
"If we can't make up the numbers we will have to move to a different way of fighting," Forbes said, adding that the current strategic mindset revolves around "one-on-one" submarine combat.
"In the future, submarines may have to fight more like an aircraft carrier," he said, describing a system in which submarines are complemented by a variety of unmanned platforms.