Nothing seemed out of the ordinary 50 years ago when Air Force Capt. John Haug and his crew took off in a B-52 carrying four hydrogen bombs.

It was the height of the Cold War, the era of “Dr. Strangelove.”

The Stratofortress departed Plattsburgh Air Base in Upstate New York on Jan. 21, part of an ongoing airborne alert program under the Strategic Air Command and satirized by Stanley Kubrick’s 1964 classic, subtitled “How I Learned to Stop Worrying and Love the Bomb.”

The mission had a name the film’s Gen. Buck Turgidson would have endorsed with a grin, “Operation Chrome Dome,” but there was nothing humorous about the underlying rationale.

“The justification was the threat of a Soviet surprise attack,” according to Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists. The idea was to protect the Air Force’s capacity to respond to a nuclear attack by keeping a certain number of nuclear-armed bombers in the air at all times.

As many as 12 planes loaded with nuclear weapons stayed in the air around the clock.

“They flew from their bases in the United States in formation via several routes that changed over the years into the Mediterranean, over Greenland and Canada, and into the Pacific,” Kristensen wrote in an email.

Despite Haug’s cataclysmic cargo — Kristensen estimates it packed a payload 293 times more powerful than the atomic bomb dropped on Hiroshima at the end of World War II — there appeared to be no reason for alarm.

Air Force bombers had been flying Chrome Dome missions since 1961. Haug’s crew had trained together and included an extra pilot to allow the primary crew to get some sleep during the 24-hour mission, according to an account of the flight published by the Air Force in early 1970.

The flight started uneventfully.

An initial aerial refueling four hours into the mission went smoothly. An hour later, Haug ordered his co-pilot to get some shut-eye and brought the extra pilot into the now-vacant seat.

The trouble started a short time later.

Temperatures in the cabin grew uncomfortably cold. The airmen overcompensated by turning the heat up as high as it would go, leading to complaints that the plane was too hot, according to the Air Force.

After the climate controls were reset to a cooler temperature, someone reported the smell of burning rubber. Haug ordered crewmembers to don oxygen masks as they looked for the source of the odor.

They eventually found it — a fire under a metal box in the lower crew compartment.

Haug radioed the U.S. air base at Thule, Greenland, located on the far northwest of the gigantic icebound island, for permission to land. The Air Force told The New York Times’ Neil Sheehan, that 14 minutes after the fire was detected, six of the crewmen successfully ejected from the plane. A seventh died.

The abandoned bomber continued on its way before slamming into the ice-covered waters of North Star Bay, less than 10 miles southwest of Thule.

Search crews rescued the survivors from the winter cold that hovered between minus 18 and minus 25 degrees, but the bombs were nowhere to be seen.

“The condition or location of the weapons that were aboard the aircraft is still unknown,” a National Military Command Center Memo noted Jan. 22.

“We used up all our firefighting equipment, and we even tried to smother the fire, but the smoke got so bad that I had no choice but to bail out my crew,” Haug said at an abbreviated news conference reported by the Times. Haug declined to answer when asked if he had ever flown over Greenland in a nuclear-armed B-52.

Long-standing Danish policy prohibited the presence of nuclear weapons anywhere on its territory — and the Danes were kept in the dark by the United States about the nuclear-armed flights over Greenland, Kristensen has written for the Nuclear Information Project.

Alarming as it was, the crash near Thule was not an isolated incident. By 1968, crashes of bombers carrying nuclear weapons had become old news.

Two years earlier, another B-52 carrying four hydrogen bombs crashed near the Spanish town of Palomares on Spain’s southeast coast after colliding with a refueling tanker.

One of the four bombs was recovered virtually intact, according to The Washington Post, but two others released plutonium when their explosive triggers detonated. A fourth bomb fell into the sea, prompting a search that involved thousands of uniformed personnel, millions of dollars and 20 ships that ranged from submarines to minesweepers.

The fourth bomb was eventually recovered, but radiation released by the damaged weapons required a U.S.-led effort to decontaminate 640 acres of topsoil in the farmland around Palomares, Tad Sczulc reported in the Times.

Military personnel initially wore face masks and gloves as they removed the soil, according to Sczulc’s account, and more than 1,600 tons of contaminated soil were sealed in steel drums and shipped to the U.S. for disposal.

In 1964, a B-52 carrying two unarmed atomic bombs crashed in western Maryland.

In 1961, two nuclear-laden B-52s went down — one near Yuba City, Calif., and another north of Goldsboro, N.C. The bombs survived both crashes without detonating, but a 2013 report in The Guardian said researchers had found government documents indicating that safety controls on the North Carolina flight were inadequate.

“One simple dynamo-technology low voltage switch stood between the United States and a major catastrophe!” according to the document.

In the aftermath of the Thule crash, policymakers — preoccupied by Vietnam and the seizure of the USS Pueblo by North Korea — assured each other that everything was under control.

When Defense Secretary Robert McNamara briefed President Lyndon Johnson by phone Jan. 22, his message was positive. “I am told by our people, inclusive of (Air Force Secretary) Harold Brown and others who are familiar in detail with nuclear technology, there’s no danger from this,” McNamara said.

Johnson acknowledged McNamara with a brief “good” and the conversation changed to the situation around Khe Sanh.

On the ice in Greenland, however, the situation seemed decidedly more serious. Air Force personnel battled subzero temperatures and the sunless Arctic winter to search for the wreckage and hunt for the bombs.

“Even if it had been decided to crash a Stratofortress and its thermonuclear weaponry deliberately, more rugged search and recovery conditions could hardly have been achieved,” Sheehan wrote. “In this atmosphere, ears exposed for 10 minutes develop third-degree frostbite, and if left uncovered much longer, freeze entirely. “Ballpoint pens do not function, certain types of paper become brittle and crumble, and instrument batteries go dead in a few minutes.”

The Air Force dubbed the recovery effort “Operation Crested Ice,” but according to Sheehan, shivering cynics gave it another name — “Operation Dr. Freezelove.” As the hunt for the bombs continued, searchers encountered radiation and located pieces of the weapons.

“The number of people involved” in the recovery effort, according to a short Air Force film, “grew from the original 68 to some 562 at the peak of operation.” Trucks, bulldozers and other heavy equipment “plowed, piled up and loaded the contaminated crust and debris into large steel containers for storage, shipment and final disposition,” according to the film’s narrator.

The Danish government barred the local Inuit population from hunting within the crash zone or boiling meat with melted seawater. The Air Force eventually filled 67 25,000-gallon tanks with contaminated snow, according to Maj. Gen. Richard Hunziker, the Air Force commander who supervised the cleanup and recovery efforts.

On Jan. 28, the Air Force reported parts of all four bombs had been recovered as well as radiation levels that suggested the TNT components of the bombs had exploded during the crash, according to Sheehan. The cleanup continued — and the aftermath of the crash lingered for decades.

In the mid-1990s, as recounted by Kristensen, Denmark learned from declassified U.S. documents of the plane’s real route and position at the time of the crash. Public outrage intensified when the Danish government discovered that, despite Danish policy, the United States had stored nuclear weapons in Greenland.

Events at Thule returned to the headlines in 2008 with a report by the BBC that — despite U.S. claims to the contrary — searchers were only able to locate three of the four bombs.

A Danish inquiry challenged the BBC’s findings, but anxieties about potential nuclear contamination remained.

In 2010, the Guardian’s Stephen Pax Leonard wrote that unresolved concerns of nuclear contamination from the Thule accident represented one of several threats to the way of life of native Eskimo hunters.

While the Thule crash left political problems and environmental concerns in its wake, it resulted in a significant policy shift that would have no doubt disappointed the bomb-happy general of Kubrick’s dark comedic masterpiece.

“The day after the crash,” according to Kristensen, “the chairman of the Joint Chiefs of Staff ordered an end to the program of flying nukes.”