WITH MOUNT St. Helens' recent eruptions in mind, can you imagine the eruption of a volcano so massive and so violent that it let the sea into the crater? And that someday tourists would enter the crater aboard a large cruise ship?

It's not fiction or fantasy. My wife and I sailed into a volcano's crater in the Aegean Sea.

The remains of the volcano, an island known as Thera (or Thira) in antiquity, is now usually called Santorini. It forms one of the Greek islands known as the Cyclades and is 70 miles north of Crete. And that closeness to Crete, home of the Minoan civilization that seemed suddenly to die some 1,500 years before Christ, is part of the extraordinary story of Santorini, which I will get to in a moment.

I had never paid much attention to our earth's more than 500 active volcanoes, although we had driven up Italy's Vesuvius and visited Pompeii, destroyed in 79 A.D. in what surely was the most famous eruption in recorded history (Pliny the Younger left us a first-person account). We were on Mount Agung in Bali a few months before it blew in 1963; we've seen Manua Loa spouting on Hawaii; and we had read about Krakatoa, the Indonesian volcano whose eruption in August 1883 (well-recorded by the Dutch who then were the colonial masters) created a tidal wave that washed over parts of Java and Sumatra, killing some 36,000 people. Volcanic dust from Krakatoa circled the earth, darkened the skies over London and produced spectacular sunsets in many places for months after the eruption.

But we had never even heard of Thera, or Santorini. Now I can assure you that there is no more spectacular sight to be seen anywhere in the world, and I've seen most of them.

As in the case of Mount St. Helens, in Washington state, earthquakes frequently accompany eruptions. Santorini is earthquake-prone; as recently as 1956 a quake, in less than a minute, smashed 2,000 houses on what remains of the island. And it's not all that impossible that the volcano might blow again; smoke has curled up from one of its two remaining cones, islands within the crater, more than once in the past couple decades.

Our vessel was the Epirotiki Line's Argonaut based in Piraeus, port of Athens, a 4,500-ton motorship chartered by the 101-year-old Raymond & Whitcomb Co. of New York. The travel company designs and operates programs for museums, private clubs, colleges and alumni groups across the country. We had signed up for Swarthmore Alumni College's seven-day cruise from Athens that visited Samos, Myconos, Thera (Santorini) and Istanbul, along with other ports of call.

As you approach Santorini by boat (there now is an airstrip so you also can fly there from Athens), you think you see a rim of snow on the highest lip of the crater created by the eruption. In fact, it is a fringe of whitewashed homes, shops and churches. Then you sail into the crater, about nine miles long and six wide, through one of the breaks in the volcano's almost circular wall that let in the sea.

It is now well established that Santorini exploded around 1500 B.C. And it was the theory of the Greek archeologist who excavated the ruins of a town, called Akrotiri, that the eruption produced a vast tidal wave which, he reasoned, reached Crete in only half an hour, devastating that big island's north coast. It was that disaster, archeologist Spyridon Marinatos concluded, that ended Minoan civilization. There are, of course, rival explanations. Two years before he died in 1974, Marinatos wrote about this in National Geographic; he is buried amid his archeological excavation, or "dig," at Akrotiri where they will show you his grave.

At many digs, among them Pergamum on the Turkish coast which we saw on this same trip, you have to let your imagination run in order to visualize what man had wrought there long ago, so great has been the ensuming pillage and destruction. At Santorini, Marinatos protected his dig, begun only in 1967 at a spot first touched by French archeologists, with a vast corrugated roof over the village he uncovered, and where work still goes on.

Here it is not very difficult to visualize life on the island during the second millenium B.C., when Minoan civilization spread out from Crete across the Aegean. (We have yet to visit Crete and for anyone going to Santorini it would make sense to first see Crete.)

What you see at the excavation, dug out from under as much as 100 feet of pumice and ash (just as Pompeii was dug out), is a small town of perhaps several hundred people, not unlike the modern towns on the island. There were narrow irregular cobbled streets, with a drainage system collecting the waste water from plumbing in the two-, three- and perhaps four-story houses built of stone and mortar with wooden joists. (In the reconstruction, the wood, burned out at the time of the eruption, has been replaced by concrete beams painted to resemble timber.)

Casts have been made from the "negatives" of tables, beds and chairs by using plaster to fill the space left by the burned wood. Dug out were ceramics and some very large jugs for storage of olive oil, wine, grain and so on.

But at Santorini no jewerly and only a few metal artifacts have been found.

Nor are there any of those extraordinary human remnants one sees at Pompeii, in the form of plaster casts filling the hollow spaces left by the consumed bodies of people trapped by Vesuvis' ash and fumes. Only a few animal remnants have turned up.

Why? Quite evidently, the people on the island had enough warning to sail away; Marinatos believed that one or more earthquakes had preceded the eruption. Who and what was lost at sea no one knows.

What remains, and what was discovered at the dig to delight the visitor's eye, are the strikingly modern frescoes of humans, flowers, birds and beasts, and a fleet of ships.

At the dig on Santorini you see the rooms where these plaster frescoes lay buried for 3,500 years or so, but you have to go to the National Archeologitcal Museum in Athens to see the real thing. They were painstakingly pieced together from crumbling fragments in ochre, black, blue, yellow and green, all in vivid hues. Both painting and pottery reflect elements of the Minoan are to Crete. After all, 70 miles was not so much of a sail; the Mediterranean abounds in evidence of travels in much earlier eras.

We know that in many primitive civilizations gods were thought to dwell in volcanoes, with eruptions signaling their displeasures with man. All sorts of folklore has been passed down to us. Santorini has its own tales.

In his Dialogues, Plato described what we take to be the mythical island of Atlantis, believed by the ancients to have been sunk beneath the seas by an earthquake. Perhaps, as one writer put it, Plato, "presenting the paradigm of his ideal state, utilized information he had concerning the Minoan civilization and the reuption" of Santorini. Perhaps. Relating Santorini to such phenomena as the parting of the Red Sea and the plagues of the Pharaoh (gases and dust reaching Egypt from Santorini) are still more farfetched.

Today several thousand Greeks live rather daringly on what remains of Santorini, mining and exporting volcanic ash used to make cement (ferdinand de Lesseps used it in protecting the walls of the Suez Canal in 1866), cultivating grapes for the local wine and servicing the tourists. As many as 18 cruise ships have been counted within the crater at one time -- not anchored, the water is too deep, but tied up to buoys. You may reach the top of the island nowadays by bus up a steep road, but to come back down to the dock requires either walking down 587 steps in cobbled switchback or riding on the island's sure-footed donkeys.

It is all charming and peaceful and a magnificent sight to come upon. But so were the slopes of Vesuvius -- and the lovely land around Mount St. Helens.