The Black Hills, S.D. — Tucked up near a corner of Clyde Ice's 160-acre farm, just off Interstate 90 and surrounded by alfalfa, is a three-foot-thick slab of concrete. There are a few electronic gadgets nearby, and the 11/2-acre site is surrounded by a chain-link fence.
But the traveler headed for Mt. Rushmore probably wouldn't notice it. Unless somebody pointed it out, he wouldn't know that beneath the 90-ton slab, in a hole about 10 feet across and 60 feet deep, is an intercontinental ballistic missile armed with a very large nuclear warhead and aimed at the Soviet Union, about 30 minutes' flight time away.
There are 150 Minuteman II missiles spread over 13,500 square miles of South Dakota, part of the 44th Strategic Missile Wing based at Ellsworth Air Force Base near Rapid City. With the turn of four keys, any or all of them would thunder skyward as they reached for their 700-mile apogee, undoubtedly singeing the alfalfa on the way up.
At least some of them would, to put it more accurately. Officers at Ellsworth say that 98 percent of their missiles are typically on alert status, cocked and ready. But they add that perhaps only 5 percent would survive a Soviet first strike, before which they would not launch. And critics have raised questions about the reliability of the Minuteman IIs, which have not been successfully tested from an operational silo since the 1960s.
Col. Robert Ceruti, the missile wing commander, likens the weapons he oversees to old cars that are continually refurbished. They may now have harder silos and improved targeting systems, he says, ''but the fact is, the technology still is old.''
''Can you be certain tomorrow if you turn the key it's going to happen?'' he asks. ''The only way that question is ever going to be answered is to turn the key.''
This is why every administration since President Nixon's has pressed for building and deploying the new MX missile. The Air Force has been jerked back and forth by Pentagon civilians and the Congress on basing of the MX, a 10 -warhead behemoth more than twice the size of the Minuteman. It may end up in Minuteman holes, at least temporarily.
But that's a question for Washington to decide. In South Dakota, the concern is how to remain a strong part of the US nuclear triad.
Maintenance personnel and crews from Ellsworth drive half a million miles a month to service and man the missiles and their launch facilities. Colonels may command, but it is younger men (and, increasingly, women) who sharpen and carry the spears in this potentially deadly confrontation. The typical technician is 21 years old, the typical missile crew member, a young lieutenant.
Each launch control facility, manned for 24-hour periods by a commander and deputy, oversees 10 missiles. These facilities are buried 50 feet underground no closer than three miles from a missile. They are bomb shelters, hardened with concrete and placed on ''shock isolators'' to absorb the potential blast of incoming warheads. They are stocked with food and equipped with their own power supply and water purification systems.
''We're prepared to stay here two weeks after a nuclear war,'' says 1st Lt. Mark Johnson, a missile commander and instructor from Black River Falls, Wis.
''Basically, we're just baby-sitting the missiles,'' adds his deputy, 2nd Lt. Evan Hoapili, a recent University of Colorado graduate.
Safeguards abound in the missile launch system, say those with their fingers near the triggers. Nothing would happen without coded orders from the National Command Authorities (the president and secretary of defense or their successors). Two officers in each of two separate launch facilities must independently activate a series of controls in proper sequence, culminating with the turn of separate keys - a total of four ''launch votes'' - before a missile can blast off.
''This is a very redundant and very safe system,'' says Lieutenant Hoapili. ''We have people who give us checks, and the people who give checks are checked.''
The missiles themselves each have six preprogrammed targets, which are changed from time to time. Powered and inertially guided flight lasts just three minutes, after which the 1- to 2-megaton warhead follows the course of gravity and weather. Still, it has a 50-50 chance of landing within a quarter-mile of its target. The newer Minuteman III missile carries three warheads that are smaller but more accurate. In either case, everything must take place in proper sequence and with precise timing for the boom to occur.
''If any little thing goes wrong, the warhead won't be armed,'' says Master Sgt. James Doll, a 17-year veteran in charge of missile maintenance training at Ellsworth. ''As far as I'm concerned, these missiles are very reliable and very safe to operate.''
These days, strategic experts and the public are questioning America's nuclear doctrine and weaponry. Many are asking if the days of the land-based ICBMs are over. Someday, perhaps, the Minuteman IIs here will be retired in favor of newer weapons or - better yet - disarmament.
That would be just fine with Clyde Ice, a man in his 90s who claims he was South Dakota's first licensed pilot and who has a great-grandson at the US Air Force Academy.
The government paid him $3,500 for the acre-and-a-half in 1962, but he lost 1 ,200 feet of road frontage and that's lowered his property's value considerably. He still wishes the Air Force had dug the hole right next to the fence.
''If they had put that missile up in the corner, it wouldn't have bothered me half as much,'' he says. ''All these years I've had to farm around it.''