The Lost Continent Found : UNEARTHING ATLANTIS; An Archaeological Odyssey, <i> By Charles Pellegrino</i> , <i> Random House: $22.95; 289 pp.</i>

It was autumn in the deserted island city of Thera in 1628 BC, with a squall blowing east to west across the Mediterranean when the volcano finally erupted. In a nanosecond, according to Charles Pellegrino in “Unearthing Atlantis,” the course of civilization forever changed.

Thera, the ancient name of the Greek island now called Santorini, became an inferno, then collapsed at its center, leaving a quiet bay where there once had been a towering mountain. Minutes later, a tidal wave 80 feet high rose to inundate Crete, the heart of Minoan civilization, 70 miles away. Volcanic ash filled the air for days as far away as Egypt and Turkey.

As Thera disappeared, more than one legend was born. Greeks, Egyptians and Hebrews explained the fallout from the eruption--an explosion equal to the simultaneous detonation of 150 hydrogen bombs--in the idiom of myth and legend. Pellegrino sifts through this ancient record and brings to it fresh data from underwater explorations and studies of more recent volcanic eruptions, including the scientifically monitored eruption of Mount St. Helens in 1980. He examines them all in light of the excavations under way near the town of Akrotiri, on Santorini. The result is a convincing case that Thera is the legendary sunken city of Atlantis.

Pellegrino, a Jack of many trades described as a “astrobiologist/paleontologist,” is a spell-binding storyteller. The discovery of whole streets of houses beneath the hardened tuff on Thera, he maintains, is the greatest archeological discovery ever, greater even than the opening of King Tut’s tomb. For the tomb was a single room, while beneath the lava-encrusted ruins on Thera lies an entire city. So far, 10 buildings have been slowly, painfully excavated--only two completely--revealing brilliantly colored, sophisticated frescoes, household furnishings and impressive hydrotechnology.

The dig, and especially the frescoes that depict the island as it was before the eruption, confirm Plato’s description of the lost city. Plato recalls that the Atlanteans “had fountains, one of cold and another of hot water, flowing in gracious plenty; and they were wonderfully adapted for use . . . also they made cisterns, some open to the heavens, others roofed over . . . the remainder was conveyed by aqueducts.” The frescoes depict the volcano, which the Therans seemed to have tapped as a source of heat, at the center of the island. They also show fountains and pools, while the excavations reveal “walls and streets literally honeycombed with pipes” providing running water and flush toilets to four-story buildings.

But as important as the discovery of this “suburb” of Minoan civilization was, Pellegrino reminds us that Minoan civilization did not exist in a vacuum. Minoa flourished at the time Stonehenge was constructed, and at Stonehenge beads manufactured in the Middle East have been found. Whether the traders who brought these beads to Britain were Minoans we may never know, but people identified as Minoan are mentioned and depicted in Egyptian hieroglyphics, and they cease to appear after about 1650 BC.

Pellegrino presents vivid accounts of shattering earthquakes that preceded more recent volcanic eruptions to explain how much warning the Therans probably had (apparently plenty, for the city appears to have been evacuated before the end), but we know little of how sudden and unexpected were the subsequent fire storms and tidal waves. No one could have anticipated the force of the final explosion.

Pellegrino, by training a paleontologist, takes us hop-skipping through geologic time, back to the formation of the Earth, to the creation of land masses and the movement of continents. Above all, he respects the volcanoes--the foundries of life on this planet--many of which he has examined with Robert Ballard (the discoverer of the Titanic) in Ballard’s Argo submarine explorer.

“Unearthing Atlantis” also takes us back through history to the eruption of Mount Pelee in 1902 on Martinique and the 1869 visit of Suez Company workers to Thera, where they excavated volcanic rock for the canal and destroyed in the course of their digging ancient Theran artifacts as well as the only ancient human skeleton ever found on the island.

The modern dig is another kind of adventure, a human tale laced with the politics of 20th-Century Greece. Here, Pellegrino interviews Nanno Marinatos, the archeologist daughter of Spridon Marinatos, who supervised the first major effort to excavate ancient Thera. The city, as it was first re-exposed to daylight in 1968, is Marinatos’ story. The current dig, ill-funded and undermanned, belongs to Christos Doumas. A onetime disciple of Marinatos, he is as obsessed with and devoted to the project as was his late mentor.

But, beyond our fascination with antiquity, what is so special about Thera/Atlantis? To Pellegrino, it marks the most powerful might-have-been in the history of the planet. Thera was a technologically advanced civilization for its time, he explains, but not modern. “Those who believe that the Atlanteans possessed electric generators, submarines and airplanes may be disappointed. (The Therans had) inlaid daggers, flush toilets and four-story buildings run through with sophisticated plumbing but not the slightest hint of electrical wiring.”

Yet Pellegrino believes that civilization went amok with the loss of Atlantis. He would have it that if there had been no cataclysm, technology would have advanced faster: “If Thera had not exploded, there might have been television by the time of Christ.”

Like others who have written about the Minoans, he is impressed that in their frescoes, and in the ruins of Thera and the palace of Knossos on Crete, there are no weapons, no signs of battlements, no locks on doors, an apparent equality of the sexes and a premium on technology. Pellegrino bemoans the loss of Atlantis, but he does not lump Thera with other mysteries. Above all, he rejects “New Age enthusiasts” who link Atlantis to the pyramids of Egypt and Mexico, nonexistent Martian pyramids, UFOs and the Bermuda triangle.

To Pellegrino, Atlantis is unique, a symbol of unfettered technology and idealized capitalism. “In the Minoan world,” he writes, “every island had the Aegean for a moat. Competition was inevitable, universal control virtually impossible. There was no reason for stagnation ever to set in.”

Equating advances in technology with advances in culture itself, Pellegrino dismisses the entire Middle Ages as a time when “nothing of consequence was being invented.” So for Pellegrino Atlantis becomes a paradise lost: lost not for any misdeed of its inhabitants, but for an explosion as unpredictable as the meteorite that vanquished the world of the dinosaurs.

Agree or disagree as you will, hours spent with this exceedingly well-written book are like a visit with a brilliant conversationalist in whose company you may be provoked, but never bored.