Buried here in the continental heart of the nation is a dimly lit place whose existence may help explain why the dreams of sleeping children sometimes become nightmares.

This is the war room of the Strategic Air Command (SAC), a heavily guarded vault beneath the well-manicured front lawn of SAC headquarters at Offutt Air Force Base outside Omaha.

The underground command center has been portrayed in so many books, movies and television scripts in the past 25 years -- as one place where The Button would be pushed -- that it has become one of our thermonuclear age's most casual cliches. Indeed, it seems eerily familiar to almost any visitor.

The room is an air-conditioned expanse about 90 feet long, 30 feet wide and 30 feet high, filled with computer screens, giant wall displays, glowing consoles and banks of telephones. A complex communications system connects this vault with long-range radar stations and other vaults around the country, or with aerial command posts that fly high in the Carl Sandburg blue that shimmers above the midcontinent in early summer.

A narrow, glassed-in mezzanine runs across the back of the room, allowing senior commanders to peer down and preside over the war room's operations. The console and center chair in the mezzanine, mundane as any office swivel-back, are reserved for SAC's commander-in-chief -- CINCSAC, in military parlance.

Most of the time, CINCSAC is not here. He is in his office in the headquarters building upstairs, or inspecting the troops, or giving speeches and testimony in support of a strong defense budget. Regardless, the command center is manned around the clock.

Soft-spoken Air Force officers cluster and confer quietly while staring at banks of signal lights that convey the combat readiness of the command's strategic bombers and buried missiles, the nuclear forces that protect 224 million Americans from attack.

SAC's 1,000 missiles and 320 bombers are armed with thousands of hydrogen bombs, most of them aimed at the Soviet Union, where 275 million people live. SAC warheads contain enough explosive force to obliterate much of life and industry there.

The USSR can do at least the same to us. Its arsenal numbers more than 2,000 strategic rockets in silos and submarines, with thousands of warheads aimed at the United States. Depending on its origin, a lethal missile attack could arrive 15 to 35 minutes after launch. Any presidential decision on how to respond to an attack likely would be carried out by SAC bombers and missiles and the Navy's missile-carrying submarines.

SAC, founded in 1946, is succinctly described by its public affairs office:

"SAC's primary mission is to deter war through its ability to deliver the majority of this nation's nuclear firepower to any part of the globe. Should deterrence fail, SAC maintains the capability to destroy the enemy's war-making powers and will to fight by delivery of both conventional non-nuclear and nuclear weapons."

The team of Air Force officers on duty one recent day proudly demonstrated the speed with which they could determine their combat units' readiness. Within about three seconds every unit across the country had responded with a signal. Capt. Robert Kennedy, the briefing officer, called this a typical response time.

The current CINCSAC, Gen. Bennie L. Davis, has spent much of his career concerned with Soviet military power and the extent of the Kremlin's will to use its weapons.

"The whole rationale is that no one uses those nuclear weapons," Davis said. "It almost gets wrapped up in a death wish. My objective is never to use them . . . . "

Visitors to the underground command center may occupy the same mezzanine chair used by CINCSAC. From the console, it is all too easy to pierce the invisible membrane of what we believe to be reality and find the room suddenly pitched headlong toward an imaginary Armageddon.

The giant screens suddenly fill with points of light, the red warning beacon flashes. Davis and Kennedy and all the others of the SAC command center rush to their places and . . . .

The dream becomes a waking nightmare . . . .