Thomas WATKINS - Ask a "missileer" - one of the key-keepers for America's nuclear bombs - whether their first career choice was to spend days on end cocooned underground in a steel bunker, and the answer is usually no.

Decades after the Cold War, and frequently forgotten by a public preoccupied with the threat of terrorism, the United States still fields hundreds of Minuteman III Intercontinental Ballistic Missiles (ICBMs), dotted in silos across rural America.

The crews tasked with the unthinkable job of actually launching the Doomsday devices have been on a state of constant alert since 1963 - a posture unlikely to change for decades to come, as countries including Russia, China and now North Korea build up their nuclear arsenals.

Most airmen join the Air Force to fly, but here on the chilly, windswept plains of North Dakota, 50 miles (80 kilometers) below the Canadian border, crews work around the clock practicing for a launch they hope will never happen.

It's not glamourous. Two-person crews pull 24-hour shifts in a launch control center buried deep underground.

The centers were built in the 1960s. Stepping into one is like a journey through time, with bulky, analog machines blinking in the crammed control room and an interior design that's not changed for decades.

The launch crews live on a diet of burgers, fried potato nuggets and other popular American foods, and there's nowhere to shower or work out.

In the event of extreme weather - not uncommon in North Dakota's harsh winters - the teams have to stay for extra shifts, sometimes getting stuck underground for 72 hours straight. They take turns sleeping during the frequent down times.

Perhaps unsurprisingly, morale has sagged in recent years, and the Air Force has been working to address a slew of quality-of-life issues.

All-night fitness centers have been built on the main base, pay has gone up and the military is even offering improved childcare programs for missileers' kids.

'So sorry for you'

Over the next 20 years, the US Air Force will switch out the entirety of its Minuteman III fleet with an as-yet-unnamed new missile known only as the Ground Based Strategic Deterrent (GBSD).

The Air Force estimates the cost of the GBSD, to be introduced late in the 2020s and phased in over the following decade, will be around $86 billion over the missiles' life cycle of about 50 years.

Lieutenant Erin Powell, who spoke to reporters Monday during a visit by Defence Secretary Ashton Carter to a "Missile Alert Facility" in the middle of a flat expanse of farmland, said joining the nuclear force was not her preferred career path, but she has come to appreciate her role.

"It's very humbling," she said, speaking of her first 24-hour alert, when she realized the gravity of her job. But "when I first got my assignment, people patted me on the back and said, 'I'm so sorry for you."

The negative connotations of missile work began after the demise of the Soviet Union, as the mission gradually received a lower priority and a less-exciting career path.

"I realize it feels at times that most people don't often think about your mission - which I know can be frustrating, even though in a way it's also a good thing, because it means you're doing your job," Carter told troops at nearby Minot Air Force Base.

A series of embarrassing revelations in 2014 described the state of the nuclear force, with dozens of airmen disciplined for cheating on proficiency tests. Other investigations have probed drug use.

Separately, some senior officers were fired for personal misconduct, including the head of the ICBM force after he went on a drunken bender during a trip to Russia.

The Pentagon in 2014 unveiled an "action plan" that included reorganizing the command structure, restoring "pride" in the mission and more funding for equipment and additional personnel.

Air crews are now subject to less-frequent proficiency tests and are instead allowed greater autonomy in ensuring their own skills are up to scratch.

"It's a healthy environment," said squadron commander Lieutenant Colonel Jared Nelson, who also admitted that nuclear missile work wasn't his first career choice.

"I didn't pick this career field either, but I've had a better family life than colleagues who got what they wanted," he said, referring to the relative stability of the work, compared to pilots who deploy across the world.