The instant a major nuclear war began, the moment the music died, the world of man would be changed irreversibly. So would the world of nature.

Somewhere over the midwestern United States, the Looking Glass Plane, the flying command post of the strategic wing of the Air Force, would be aloft as it has been without interruption since Feb. 3, 1961.

But the world beneath and above the Looking Glass would be altered in a way that is far from fully understood even now, 37 years into the nuclear age.

Gone would be the bunker world into which man had burrowed to protect himself. Gone would be national leaders. Gone would be man-made structures ranging from communications satellites above to their ground links and radar dishes below. Gone would be computer memories and simple telephone lines. Also gone, or badly mangled, would be AT THECRATER'S EDGE PART 6 some of nature's structures that men take for granted in their world of modern communications. Nuclear detonations punch holes in the ionosphere, off which men bounce high-frequency radio messages. X-rays, gamma rays and careening neutrons would foul the atmosphere through which modern man talks. Electromagnetic pulse would destroy the tools of his communication.

The wheels of war would have begun, but, quite possibly, the wires of communication would go dead. Troops might go deaf to commands, enemies deaf to negotiation. A nuclear war would be considerably easier to start than stop.

"The precariousness of command channels," John Steinbrunner, a Brookings Institution specialist, said in 1978, "probably means that nuclear war would be uncontrollable, as a practical matter, shortly after the first tens of weapons are launched."

Uncontrollable.

In less than 30 minutes after the first alert emerged from the bowels of Cheyenne Mountain, Mike Howe would bank his aircraft over the Great Plains, holding the speed down to about 400 mph to conserve fuel.

Howe, 36, a Strategic Air Command careerist from Spencer, W.Va., already would have broken the seal on the cabinet a few feet behind him. From the cabinet he would have taken gray flash screens and fitted them tightly over all the cockpit windows in the Looking Glass Plane.

Farther behind him, near the general's seat, a small black box known as HARDS, for High Altitude Radiation Detection System, would light up, set off a piercing noise and begin a printout with details of the "event" the Looking Glass was nearing. Howe would turn off the air intakes to the converted Boeing 707.

He would think of his family and friends on the ground. Perhaps he would think of the twist of fate that had placed him aloft on what, until minutes ago, had been just one of the dozen routine eight-hour shifts he spends in the pilot's seat each month. But he would have work to do.

So Howe would reach for a black eye patch to protect one of his eyes in case another "event" was about to occur. Then he would peel back the small, square peephole in the protective flash screen. He would peer out into the new world, trying to find a way out of the clouds HARDS told him he had entered.

Radioactive fallout clouds are not good for living beings, neither animals nor children, neither Soviets nor Americans, neither civilians nor the battle staff of 20 men and women sitting behind him.

The clouds would be pulsating red.

On Dec. 6, 1941, as Washington attempted to warn Hawaii of a possible attack, an alert message was transmitted by high-frequency radio. The message was delayed for hours by natural disruptions of the ionosphere, solar flares and the like. It arrived after Japanese planes had swept out of the early-morning sun and shocked America into the Second World War.

The United States now has 43 different communications systems for use in war, including those that use the ionosphere and those that don't. They range from Very Low Frequency, which operates by trailing a five-mile copper-wire antenna out of the back of the Looking Glass, to the elaborate maze of wires and satellites run by the Bell System.

But man has come a long way since Pearl Harbor. The bombs used to end that war not only disrupt the ionosphere more effectively than does the sun, they also create a phenomenon known as electromagnetic pulse (EMP), a surge of energy far more powerful than lightning.

Scientists are uncertain about exactly what EMP will do. They were only vaguely aware of its existence when the United States and the Soviet Union ended atmospheric testing of nuclear weapons. Some say they believe that a single, high-altitude explosion over the Midwest could knock out most communications in the United States. A similar explosion, or several, would do the same thing to the Soviet Union.

EMP would cripple electrical-power stations, its energy overwhelming surge arresters designed for lightning. It would burn out fuses, deaden intercoms, silence alarm systems. Copper wiring and aluminum aircraft bodies are EMP collectors. Among the most vulnerable of man's systems are his most sophisticated: computer circuitry, transistors and silicon chips.

Roger Molander, who was a National Security Council aide under three presidents, said all three were "scared silly" that the command system would be beyond control after weapons were launched. Larry Smith, a former Senate Armed Services Committee aide and one of Capitol Hill's most serious students of the unthinkable, said he can visualize a "worst-case scenerio" in which nuclear decisions were "being made by a second lieutenant in a foxhole."

America has spent billions upon billions on nuclear weapons and their delivery systems and has constructed intricate systems against their accidental use, but authority after authority says that once the music dies the system for controlling them may die too.

"We build 'em and then try to figure out how to control them later," said Rear Adm. Paul D. Tomb. "It's kind of a strange way to run a railroad." Tomb, a former submariner, now sits in Omaha and worries about communications and a better way to run the railroad to the crater's edge.

The Reagan administration has made the problem a central part of its staggering defense buildup, a decision that makes the planners more secure, the public more nervous. Military officials estimate it may cost $25 billion. But even that carries no guarantees.

In incredibly bleak testimony on the subject during 1980 defense budget hearings, Dr. Gerald P. Dineen, then an assistant secretary of defense in charge of command, control and communications, told the House Appropriations Committee that he would have to go into secret session to discuss the system's deficiencies.

Dineen was asked how many of the 43 communications systems would be lost "by a single nuclear explosion in the atmosphere."

"It would be hard to give you an exact number," Dineen replied, "but . . . quite a few of them." He said he didn't "know whether it would be 30 out of the 40 or something. Of course, we don't know. There are a lot of uncertainties."

Men are trying to "harden" military communications gear against EMP with super surge arresters, shielding and redundant systems. They don't know if it will work, because they can't test it without a nuclear explosion.

Inside the Looking Glass, Mike Howe, the general and his staff of intelligence, communications and maintenance experts would be in what they call the "trans-attack" stage of the war. The war would be under way but not over.

Howe is a genial, happy-go-lucky fellow who looks and sounds like a man who enjoys life and his unusual work. The Looking Glass Plane is not hardened. Howe speaks almost gleefully of the anti-EMP trick they have strung across the top of his airplane's fuselage, jury-rigged with the same elan that battle-wise pilots showed in earlier wars, outthinking the engineers.

From the tail of the Looking Glass to its fuselage, two high-frequency radio antennae have been strung. When EMP takes out one, Howe said, they'll turn on the other. He does not speak of a second surge of EMP.

The bunker world into which men burrowed decades ago with their elaborate communications devices and war-game computers would be gone. "Any fixed command center is vulnerable," Dineen testified. He meant you can't dig deep enough to escape thermonuclear weapons.

During the middle years of the nuclear age, after the Soviets had built the bomb but before they had developed intercontinental missiles to deliver them, America went underground.

In Colorado, the nation hollowed out Cheyenne Mountain to give it a secure warning post. Inside, a small city was built on shock absorbers as tall as a man, with a 25-ton blast door to seal it off from the outside and thousands of feet of granite to protect it from above.

In Virginia, Mount Weather was carved out the same way, as an evacuation bunker for leaders. Mount Weather has blue hammocks to sleep hundreds, a dining room and a hospital, desks for Cabinet officers and telephones for secretaries. It has a press room.

In Omaha, the SAC command center, with its eerie blue alarm lights, its huge missile-watching screens and its war-gaming computers, was buried under the manicured green lawn of Offutt Air Force Base. The War Room in the Pentagon went deeper, the Situation Room into the basement of the White House.

That was for yesterday. Today an accurate Soviet missile, with a medium to large warhead, would make Cheyenne Mountain a tomb, Mount Weather a gravel heap. The missiles don't need to be accurate to destroy SAC headquarters, the War Room or the Situation Room. Near misses will do.

The men and women who work in these places would have less than 30 minutes. That makes for philosophical beings. "Before the first weapon arrives," said Capt. Bob Walden, who worked in Cheyenne Mountain a few years ago, "we will have completed our mission."

The mission of a nuclear Third World War, however, presumably would not be completed.

So America's bunker world went airborne, first with the Looking Glass Plane and later with the National Emergency Airborne Command Post, one on ground alert for the president at Andrews Air Force Base near Washington and others on ground alert in Omaha.

But dilemmas replace dilemmas in the world of E=mc2.

Dineen testified to the dilemma of placing the NEACP, an elaborately refitted $250 million Boeing 747 nicknamed the Doomsday Plane, at Andrews. It is the best and the worst place for the plane.

Best because it is eight minutes by helicopter from the president and his nuclear-command designates; worst because of its proximity to the coast. The flight time of a submarine-launched missile is about eight minutes.

Dilemmas. The Andrews plane is EMP "hardened," as best as man can against something he doesn't understand fully. The Omaha-based planes are not.

Dilemmas. In Washington there are nuclear-command authorities, men in addition to the president who could turn the war on or off. In Omaha there are none, at least no legal ones, although legality would become a very fine point if thousands of megatons were loosed on the United States.

The moment Soviet warheads struck Omaha and the underground command post was "rendered inoperative," as SAC euphemistically puts it in literature that rarely mentions death, operational command of America's remaining intercontinental missiles and fleet of strategic bombers would be transferred instantly to the Looking Glass. So would whatever war scenerio, completed or incompleted, the president had ordered before he was rendered inoperative.

The Looking Glass Plane, appropriate to its double-meaning name, might have the best view in the United States of the trans-attack realm of a nuclear Third World War. Or it might have no view at all.

SAC's favorite photograph of the inside of the Looking Glass shows a smiling three-star general sitting in his swivel chair, the double-locked code box on his left, the little black radiation detector obscured at his right. He is talking on a conventional black telephone. It has five lines. The phone might be dead.

A Soviet general, sitting in a similar aircraft flying somehwere over central Asia, would have no magical formula that would make his situation more tolerable, his telephone or his ground bases less inoperative.

He would have command of the Bear and Bison and Backfire bombers headed toward America. The SAC general would have command of the B52s and the F111s heading toward the U.S.S.R. The bombers are slow. The B52s could take 10 hours to get to their targets, time for a lot of thinking about saving what could be saved, to think about calling them back.

One of the most storied devices in nuclear lore is the "hot line" between the White House and the Kremlin. The Americans and the Soviets agreed to install the line shortly after the 1962 Cuban missile crisis, during which messages between President John F. Kennedy and Premier Nikita S. Khrushchev sometimes were delayed by as much as eight hours.

It is a teletype system, with terminals in the office of the chairman of the Communist Party in Moscow and dual terminals in the Pentagon war room and the White House in Washington. It has been used in occasional international crises, including the Yom Kippur War in 1973. Military officers call the messages "nastygrams."

The hot line originally had two connecting links, one an underground and undersea cable system with a backup radio circuit. In the early days the line failed regularly. Once it was knocked out by a manhole fire in Rosedale, Md. Another time a farmer in Finland cut the cable while plowing.

The backup radio system restored service rapidly. But in the early 1970s, the two superpowers modernized the hot line with parallel satellite systems. The American ground links are at Etam, W.Va., and Fort Detrick, Md., with underground telephone cables completing the tie to Washington. The satellite ground stations are "soft" targets. In 1977 former defense secretary Donald Rumsfeld testified that they could not survive a direct attack.

The nastygrams might flow back and forth as tensions mounted, but they would stop when the first bombs fell. So would any attempts, via the hot line, to be less than nasty.

The basic American military communications system is called WWMCCS, the World Wide Military Command and Control System. WWMCCS uses virtually all of America's 43 communications systems. It ties together American military commands around the world. During a 1977 peacetime test of the system, code-named Prime Target, 38 percent of the messages got through.

During the 1967 Middle East war Israeli gunboats sank an American intelligence ship, the Liberty, in the Mediterranean. Three messages were sent to the Liberty before the attack, telling her to abandon her position. None of the messages was received.

In 1968 it took the North Koreans four hours to take the captured USS Pueblo into port after capturing the ship. Communications failures, human and technical, prevented any rescue attempt.

Seven years later the Pentagon used the Mayaguez incident, in which a freighter was assaulted by Cambodian forces in the Gulf of Thailand, as an example of how communications had been improved. Direct communications from Washington to the rescue ships aided the mission.

Once the Mayaguez was rescued, President Gerald R. Ford issued a direct communication to halt all offensive actions against the Cambodians. Thirty minutes later American planes attacked Cambodia. Thirty minutes is the flight time of an intercontinental missile.

Inside the Looking Glass, the general is aware of such nuclear imponderables as missile bias and fratricide.

He knows that missiles have been given a very high rating for accuracy, known as their Circular Error Probable. Western estimates of the CEP for modern Soviet intercontinental missiles, the silo-busters and city-killers, ranges from 1,200 feet for good ones, 750 feet for the best. That means the best missiles have a 50 percent probability of landing within a radius of 750 feet of their target.

He also knows the estimates are based on test flights on established missile ranges. He knows that missile bias--little flaws in trajectories resulting from atmospheric irregularities, winds, storms, magnetic pull--has been adjusted on the test ranges. He knows that missiles have never been fired over the polar regions, across which the Soviet missiles came at him and his went back, and that the bias is different there.

He knows that warheads double-targeting prime objectives, such as his Minuteman silos, tend toward fratricide, with one killing the other.

The last message he received from Omaha told him where the Soviet missiles were heading and where the American missiles were directed. But he does not know precisely where the missiles landed, unless he can talk to somebody. And he is the somebody. He has the looking glass.

The general is experiencing what Dineen told the House committee: "After the attack there will be so much confusion that the information which you acquire before the actual impact may be the only information you have for some period of time."

In the Looking Glass, the general might not even be certain who started the war, or why, or how.

Inside Cheyenne Mountain, America's nuclear-attack warning post, 147 false alerts were recorded in an 18-month period beginning in January, 1979. Most of them were minor, caused by the failure of a 47-cent computer chip, and were quickly identified as false alarms.

At least a half-dozen were more serious, and took American military authorities to the second of three levels of alert conferences. At SAC bomber bases the klaxons wailed and B52 crews raced to their armed planes and started engines.

In one case a technician accidentally placed a training tape in the operational computer system, and Cheyenne saw thousands of warheads rising out of the Soviet Union. The alert lasted six minutes, although the men inside Cheyenne say they knew it was a phony within two minutes. They say the media made too much of the false alarms, that the system has too many cross checks to get out of hand.

"We saw missiles coming out of missile sites where they didn't even exist," said Col. Donald Moore, a man who began his Air Force career as a pilot-observer of atmospheric nuclear tests and now works on communications inside the mountain.

The computer chips have been replaced, training exercises and their tapes moved out of the mountain and the computer-warning system overhauled. The general in charge of the mountain has testified that the errors cannot occur again.

Three years before the series of false alarms Dr. Herbert K. York, a physicist who worked on the Manhattan Project, designed better and better bombs throughout the '50s and then dropped out and wrote a book called "Race to Oblivion," testified before a different committee. He talked of an earlier false alarm, when radar sites picked up reflections off the moon and misread them as a Soviet bomber attack.

"There are two lessons to be learned," York testified. "First, these things do happen, and second, they can be fixed." He stressed that the system was new then, and added, "The next time that we make a big improvement in our detection capability is precisely the time when we will run the greatest risk of another false alarm."

York added a further warning about being too proud of modern technology: "I would advocate that one not deceive one's self concerning the perfections of the systems of this country." He said the perfections of the Soviet systems might be even more deceptive.

Inside the Looking Glass, Mike Howe ducks in and out of the clouds. As time drones on, the general ponders, but tries to suppress, thoughts of the holocaust beneath him: 20 million dead, 50 million dead, 100 million dead? His wife and family in Omaha. Dead. He also knows that people are alive, including a presidential successor or a member of the National Command Authorities. Somewhere, somehow, a helicopter swept through to a bunker missed. But he hears nothing.

He knows something about what military historians call "the fog of war," the psychological stresses that exacerbate human weaknesses in the chaos and confusion of battle, causing some men to overreact and some not to react at all.

He studied well in the war colleges, and he recalls that Gen. Douglas MacArthur, one of America's great military heroes, inexplicably waited nine hours after the attack on Pearl Harbor, allowing the Japanese to destroy his air force on the ground in Manila. He recalls that historians wrote that Napoleon was unable to give orders and unable to understand instructions at Waterloo.

So his bombers would fly on, plodding toward a date in Russia 10 hours after the music died. If he could jury rig one or more of America's 43 communications systems, he would attempt to activate the Minuteman missiles that had not made it out of their silos. He would wonder about the submarines, beyond his command.

America's submarine forces contain almost 60 percent of the nation's strategic nuclear missiles. A single submarine in the 41-boat fleet has enough nuclear weapons to destroy every major city in the Soviet Union.

They are extremely hard to find beneath the seas, but because they have the sea for cover it also is extremely difficult to talk to them. The assumption, implicitly acknowledged, is that, because of the communication problem, they could launch without command approval. They could roam for days, weeks, months, keeping things going.

TACAMO, a Navy plane like the Looking Glass, is on constant airborne alert over the Atlantic to talk to the subs. The acronym stands for Take Charge and Move Out. The plane would have as much trouble living up to its name as would the Looking Glass.

In the Looking Glass, Howe and the general and the battle staff can remain aloft for 36 hours with midair refueling, if they can find the tankers. Then the engines will run out of oil. Then they will have to circle and search and come back down somewhere in their new world.

If that new world fits the vision of T.K. Jones, the Pentagon advocate of hardening industrial plants with aluminum chips and dirt, the survivors soon will traipse back into leveled cities and dig up the machinery of a wounded civilization.

If the world fits the vision of Dr. Jack Geiger, who once ground up baby teeth to prove that fallout found its way into children's bones, the survivors will be better off looking for a medieval historian to learn the new rules.

If the late-blooming vision of retired Adm. H. G. Rickover is accurate, the world in which the Looking Glass finally lands already will have begun accustoming itself to the not-too-slow disappearance of its once-dominant species. CAPTION: THE CRATER'S EDGE: MACHINES AND THE MOUNTAIN: Pictures 1 through 3, A sentry watches a modified Boeing 747 jetliner, the National Emergency Ariborne Command Post, at Andrews Air Force Base. Some of the equipment inside the refitted (price tag: $250 million) Doomsday Plane, and the tunnel leading to the nether world of Cheyenne Mountain, from which the alert would come. Photos by Ken Fell--The Washington Post, NORAD