IF THE world blows up, it will have started here: Trinity Site in New Mexico, an isolated I spot on a flat plain in the desert, where the world's first atomic bomb was exploded 38 years ago. Hiroshima, Nagasaki, "No Nukes," the nuclear freeze--all these date back to Trinity. It would be surprising if Trinity Site had become lost to history, and it hasn't. It's still there today for all to see--but only one day a year.

On the first Saturday in October, White Sands Missile Range, where Trinity is located, opens its doors to visitors and hosts a tour of the site. Because the Missile Range is active--with frequent practice "bomblet" runs--Trinity is closed to the public the rest of the year. Nevertheless, the one day that it permits visitors, White Sands takes as many as 1,700 history-minded people to the spot where man first turned matter into energy in a flash of blinding light.

Trinity goes back to 1943, to the Manhattan Project and the scientific laboratory at Los Alamos, N. M. There, 2,000 scientists converged to work on one of the biggest joint efforts in the history of science and technology--the invention of the atom bomb.

Work at Los Alamos was directed by physicist J. Robert Oppenheimer. When it became clear that the bomb's theoretical and technical problems were going to be solved, he set out to locate a site for the test.

The site had to be isolated, for obvious reasons. It had to be flat, so that the yield of the explosion could be accurately measured. And it had to be near enough to Los Alamos so that the bomb could be transported to it in a short time and in secrecy.

The fact that the Jornada del Muerto ("journey of death") between El Paso and Albuquerque was the perfect place to test the bomb had nothing to do with its name; rather, it had three practical advantages. It was bordered on the east by a mountain range--the Oscuras--which would serve to contain some of the bomb's fallout as the mushroom cloud moved off in that direction. The area was sparsely populated, with only a handful of cattle ranchers for miles around. But the deciding factor was that the land was already controlled by the government: it was part of the Alamogordo Bombing Range.

In May 1944, Oppenheimer, together with test director Kenneth Bainbridge, drove across the Jornada and found what they were looking for. "Fat Man," as the bomb had been nicknamed, would explode--or fizzle--here.

It did not fizzle. At 5:29:45 Mountain War Time July 16, 1945, the gadget from Los Alamos exploded and vaporized every last foot of the 100-foot-tall shot tower on which it stood. It lit up the desert and flooded the mountains with a brilliant, unearthly light.

The heat and force of the fireball melted the sand below, turning it into glass. The bomb created a tremor which tossed people out of their beds in the village of San Antonio 30 miles to the west. The shock wave broke windows in Silver City, New Mexico, 120 miles to the south. The burst of light was so intense that it could have been seen from another planet.

Today, Trinity Site looks much like the rest of the desert. Surrounded by two concentric chain-link, barbed-wire fences to protect people and animals from the radiation that persists, Trinity abounds with the sagebrush and grass--and even flowers in the springtime--found anywhere in the southwest deserts.

A few signs remain, however, of the climactic event that occurred there. For one thing, the area isn't perfectly flat any longer. The bomb blasted out a crater about 800 yards wide and about 15 feet deep. Although, to lower radiation levels, this was bulldozed over soon after the test, a depression still exists and you can't, from outside, see the obelisk which marks Ground Zero at the center.

The other sign of the explosion is Trinitite--greenish beads of fused sand that are strewn across the desert floor. Looking like bits of turquoise or jade, about an inch square, they are transparent at their thin edges, like the glass of a wine bottle. These jewels in the sand are the vestiges of the nuclear explosion's violent heat.

You can see all this during the Open House tour, held this year on Oct. 1.

The tour departs from two locations: Alamogordo to the southeast, and the Stallion Ranger Center to the northwest. From these points the Army leads caravans of cars and buses, trucks and vans across the dusty flats to Trinity.

Facilities and services for the handicapped are provided.

The caravans leave at about 8 in the morning, arriving at the site an hour and a half later. The time of a visitor's stay within the fenced area is limited to 90 minutes, to keep radiation exposure to a minimum. Radioactivity there is low to begin with, however. A leaflet distributed at last year's Open House says, "The amount of radiation (from 90 minutes at Trinity) is approximately 1/15 of that received from a chest X-ray."

At Ground Zero, the shot tower's stumps give chilling evidence of the bomb's destructive force. Once a steel structure rising a hundred feet on top of concrete pilings, all that's left of it today are a few nubs of rusted iron and shattered cement about ground level. They can easily be mistaken for rocks.

On the exact Ground Zero point stands a simple monument made of the black lava stones that are found nearby. A plaque on the obelisk reads: "TRINITY SITE Where the world's first nuclear device was exploded on July 16, 1945."

Cameras are permitted here, but food and beverages are not. Also, there's a prohibition against removing Trinitite from the grounds. Still slightly radioactive, it's better left there for others to see and contemplate. Prayers for peace are offered, and a speaker addresses the audience.

If you can't get to Trinity this October, you won't have to wait another year to see it, so long as you're content to view it from a distance. To see it from afar, you can drive to the San Antonio exit of I-25, and take Route 380 east for 12 miles. From there, Route 525 goes to the missile range gate five miles to the south. These are the roads which the atomic scientists, and the bomb itself, traveled to Trinity, just days before the test.