Shortly after sunrise on Jan. 27, Seiichiro Mimata tightened the chin-strap on his bright yellow hard hat, slid the photographs of three dead men into the breast pocket of his khaki overalls and rode an elevator 800 feet down into the steamy undersea cavern where he has spent much of the past 19 years.

At 9:25 a.m., Prime Minister Yasuhiro Nakasone touched a white-gloved hand to a silver button on his desk top in Tokyo 400 miles to the south, detonating by remote control an explosion here that brought tears of joy to Mimata's eyes.

The blast pulverized a yard-thick barrier of rock, opening a narrow pilot tunnel between Honshu, Japan's main island, and Hokkaido island to the north. The event presages the completion of the world's longest man-made tunnel and the fulfillment of a 50-year-old Japanese dream.

Mimata, a craggy foreman whom younger men admiringly call "the mole," says of the snapshots he carried with him: "Many of my coworkers have been killed in accidents, died of old age or disease, and I wanted to show them we'd finally done it."

For two decades, Mimata and a thousand fellow molemen have been burrowing beneath the turbulent waters of the Tsugaru Strait, struggling to connect the two main halves of the $3 billion Seikan Tunnel. By 1985, they expect to finish clearing away the last of the porous volcanic rock, sandstone and silt, completing a submarine tunnel of 33 miles. That is roughly the distance between Washington and Baltimore and nearly twice as long as the Chesapeake Bay Bridge-Tunnel linking Norfolk and the southern tip of Virginia's eastern shore.

Typically Japanese in its technological virtuosity, the tunnel ranks among history's greatest engineering feats. When completed, it will have swallowed enough steel to build 50 Eiffel Towers and displaced enough earth to fill two World Trade Centers.

The skill and daring of the yeoman tunnelers have made contemporary folk heroes out of men like Mimata, whose exploits have inspired best sellers, popular ballads and a recent movie epic seen by millions of Japanese. The tunnelers once had to work in neck-deep water to repair a breach when large sections of the tunnel gave way and allowed 85 tons of water a minute to pour into the chamber.

Chief engineer Shogo Matsuo expected the difficulties. When the order came to begin drilling in 1964, he recalls, "I had serious doubt we'd ever be able to accomplish such an enormous task within my lifetime."

The undersea chamber, which resembles an immense rifle barrel, originally was planned to shoot a Japanese Shinkansen bullet train from Tokyo to Sapporo at speeds of 120 miles an hour. By reducing from 17 hours to five the time that it takes to travel by rail between the two antipodal cities, planners in the 1960s hoped to pull sparsely populated Hokkaido into the orbit of crowded industrial Japan.

"There has always been a fundamental desire among the Japanese to connect the four main islands and make them one," Matsuo says, pointing to the intricately detailed maps that line his office walls.

"But, our motivation was not entirely spiritual. Once we decided to go ahead, as engineers, we were forced to push back the frontiers of technology."

Despite this enthusiasm, the pachyderm pace of drilling and huge costs have led numerous critics to single out the tunnel as one of postwar Japan's great white elephants. They charge that the project's primary function has been to suck massive amounts of taxpayers' yen into a deep, dark useless hole.

The critics note that advances in cost-efficient air travel and mountainous government debts have led the authorities to suspend the bullet-train service that the tunnel was designed to accommodate.

As a result, years before the first small local train is scheduled to lumber through the tunnel, officials have embarked on a kind of "mission impossible" to find ways of making the technological marvel pay for itself.

The ideas have included plans as diverse as opening the tunnel to automobiles that would be ferried undersea piggyback on special car-carrying trains, or threading fiber-optic cables and other telecommunications lines through the tunnel. But, as project supervisor Masahiro Fujita concedes, the controversial tunnel does not have much to offer at present except that "there's plenty of room down there."

At Yoshioka, a windswept fishing village on Hokkaido's remote southwestern tip, snow-capped mountains gather in great folds around a temporary construction camp that has been here for 20 years. When Fujita arrived in 1975 to take front-line command, the tunnel was already behind schedule and intensely involved with politics.

Fujita, a short, feisty man of 57 with ruddy cheeks and a shrewd, skeptical grin, recalls, "There was a dark mood hanging over the place, and even the workers thought it would take 40 or 50 more years to finish the thing."

With careers on the line, top executives at the Japan Railway Construction Public Corp., the state-run tunneling authority, were running low on patience, Fujita says. "One day the president told me I had to find a way to speed up excavation. The next day, a senior vice president ordered me to find a discreet, diplomatic way to bring down the curtain on the project altogether."

Suddenly, the drilling got easier. By 1974, a decade of excruciating labor had brought only 11 miles of progress on the two inclined tunnels inching toward each other from opposite sides of the strait. Over the next three years, on-the-job refinements in technology and a more genial stretch of rock strata helped workmen push through 13 more miles. Bottlenecks created by tunnel opponents began to disappear.

Then came the floods. When tunnelers unwittingly drilled beyond a series of major faults in the seabed crust in 1977, large sections of tunnel wall gave way. Three million tons of muddy saltwater gushed into the chamber, enough to fill 15 giant oil tankers. After working heroically, the tunnelers eventually pumped the water to the surface.

From a yard a day, however, forward progress slowed to a foot, then six inches. It took four months to gain only 37 yards through a geological jungle of slipping layers of sandstone and sedimentary mush.

Fujita, who now oversees Seikan operations from Tokyo, suggests impishly that wiser men might have called it quits. But wedded to a do-or-die philosophy common in a society where the concerns of the individual are often submerged in a tough-minded team spirit, they tunneled on.

"Our personalities and politics may differ, but . . . we are all of one mind about seeing our job completed," Fujita said.

At 800 feet beneath the sea, foreman Mimata and his molemen inhabit a Jules Verne realm of fanciful beliefs and futuristic technology. In the sultry, stale air, tram cars packed with helmeted tunnelers hurtle down makeshift iron rails, narrowly missing giant, slow-moving bulldozers. There is an unearthly din from clanging traffic signals and thundering jackhammers.

Women are not allowed into the tunnel because, workers believe, their presence would anger the Japanese goddess who jealously guards her underworld kingdom. Thirty-three men have been killed here, mostly in traffic accidents, and 700 have been seriously injured. Others have succumbed to a black lung disease from breathing air choked with finely granulated rock.

At this depth, the pressure per square inch is 24 times that of sea level. Since freshly dug sections of tunnel tend to shrivel, something like a sloughed-off snake skin, they are immediately sprayed with a layer of concrete. Next, iron archways are bolted in place against the 36-foot-wide chamber. Down the trackbed then shudders an erector-set contraption in the shape of an inverted crescent, a platform from which a second, 40-inch layer of concrete is pumped upward and outward against the concave tunnel arch.

Like many of the 100 men under his command, Mimata is a tough piece of work. Powerfully compact, he is 48, below the Japanese average in height, with a curly mat of jet-black hair and a low, raspy voice. In a ramshackle construction building above ground, suggesting a football locker room, workers sit on hard wooden benches and listen respectfully as Mimata talks tunnel strategy, sketching the daily assault on a blackboard.

There is a hint of lameness in Mimata's stride from a time seven years ago when a conveyor belt laden with softball-size gravel snapped away from the tunnel ceiling, and, swinging wildly from side to side, caught him in the chest, crushing five ribs. He says he will never forgive himself because, when the big floods came in 1977, he was in the hospital and not in the tunnel with his men.

"I don't know anything about tunnel politics," Mimata says sharply, but without heat. "All I know is that I've put my life on the line here almost every day for the last 19 years and never for one minute doubted the tunnel was possible."

On Jan. 27, Mimata was standing on the Hokkaido side of the long, sinuous pilot tunnel amid a group of tearful, sake-drinking tunnelers waiting for the remote-control explosion that would connect it with the Honshu half. When the blast came, the light he saw at the end of the tunnel was a string of electric bulbs illuminating a similar group of tipsy, watery-eyed workmen.

"It was funny," he says of the experience. "My mind suddenly went blank and I felt lightheaded. All I can tell you is that that final act of penetration was a thrill like nothing else."

The concept of conquering the Tsugaru Strait with a tunnel was the brainchild of Japanese Imperial Army strategists in the days before World War II. When war inevitably came with the western powers, they calculated, Allied bombers and submarines might easily block the channel, cutting off Hokkaido. The Seikan Tunnel, however, occupied only a small corner of grandiose plans for territorial linkage.

"When we were allies of Adolf Hilter's Nazis," supervisor Fujita explains, "we had big plans to connect Tokyo and Berlin by rail," something he insists "was much more than a half-baked plan."

"We had already carried out feasibility studies," he says, "but in the midst of the project we lost the war and everything went up in smoke." Defeat brought Japanese engineers back to the Tsugaru Strait in 1946 when they began seriously mapping out a tunnel route.

The Tsugaru Strait is one of three strategic straits that go through the Japanese islands, providing vital access to the open waters of the Pacific to the submarines and surface ships of the Soviet Pacific fleet. Prime Minister Nakasone, known for his hawkish views on defense, has suggested that Japan would "in an emergency" allow American naval forces to blockade the straits in a bid to bottle up the Soviet Navy in the Sea of Japan.

Military analysts here have suggested that, in time of war, the Seikan Tunnel would provide a key channel for the movement of troops, tanks and missiles to Hokkaido. In an extreme case, it might be mined and destroyed. Asked by Japanese reporters recently about the tunnel's possible role in national defense, Nakasone replied laconically, "It will be useful."

Repeated core samplings taken from the seabed during the 1950s indicated a geological parfait of rock strata whorled with sedimentary siltstone, tuff, highly fissured volcanic rock and sandstone. The seeming impossibility of gouging a tunnel out of such confused and hazardous terrain made young engineers like Matsuo shake their heads in wonder.

Teams of Japanese went to the United States and Europe in search of state-of-the-art tunneling techniques. Deep in the coal mines of West Germany, they found what they were looking for: a grouting technique that would allow them to fill in underground cavities, making their own solid rock as they tunneled.

Unlike the more uniform terrain found below dry land, however, the Seikan route contains hundreds of varieties of rock shot through with large pockets of water. The Japanese, Matsuo says, were forced to improve existing tunneling know-how in "a delicate process of adjustment every step of the way."

At each new stage of drilling, the Seikan tunnelers anticipate the line of maximum advance by boring hundreds of small-diameter holes 65 yards ahead at angles of between 16 and 30 degrees. A milky soup of liquid glass and cement is then shot through this ice cream cone-shaped network of holes where it solidifies in a matter of minutes.

"We can't say the tunnel will be 100 percent safe," he concedes, "but it's probably 99.9 percent safe." At that percentage, he says, the tunnel represents a remarkable improvement on the existing ferry-boat service across the Tsugaru Strait. It has an unenviable record of accidents, including a 1954 disaster that killed more than 1,000 passengers and crew.

"You can't judge the tunnel by ordinary human standards," insists Fujita. "The human effort and imagination involved corresponds to what it took to build the pyramids. The only difference is that a tunnel is more functional that a pyramid."

Like many of Japan's hardened tunnelers, Fujita is an optimist. Japan National Railways, which must ultimately try to pay for the tunnel, has, in recent years, piled up a dizzying ziggurat of $72 billion in debts at a time when passenger and cargo traffic has declined sharply amid fierce competition with the country's domestic airlines.

The dim prospects, however, don't bother Fujita. "After all," he says, "we didn't exactly eat and drink the money away. We have built something that will last for hundreds of years . . . and realized a dream that someday we'd be able to walk to Hokkaido under the sea.

"The tiny fish," he says, aiming an old Japanese adage at tunnel opponents, "dance on the surface of the sea. How can they know the profundity of its depths?"