On Thursday, Jan. 4, an eerie structure appeared through the mist off this historic maritime city, in waters long the province of the sailing yacht, the lobster boat and the nuclear submarine.

It stood nearly 400 feet high -- far taller than the Statue of Liberty -- and resembled nothing so much as the mutant offspring of lunar module or a giant erector set.

Longer than a football field and wider than most aircraft carriers, the sixlegged, derrick-topped platform was actually part ship and part island; a self-powered industrial zone as otherworldly in its appearance as in its name: the Zapata Ugland.

Its ultimate destination lay 100 miles south of here and about 60 miles off Cape May, N.J. There, in about 450 feet of water, it will soon begin drilling the 15th oil exploration well in the undersea geolgic zone known as the Baltimore Canyon.

For nearly a year, since the U.S. Supreme Court's final rejection of environmental challenges last February, a vast river of money and machines has been flowing toward the Canyon, one of the nation's last petroleum frontiers.

Traveling with it has been an army of roustabouts and technicians, "log readers," rig welders and mud men, summoned from Tulsa and Houston, from Aberdeen and Abu Dhabi in a $1.3 billion industry-wide gamble that oil lies there beneath the ocean floor.

Already seven oil companies have spent more than $97.1 million in drilling costs alone on eithet finished wells in the Baltimore Canyon. Seven of those have been dry; the eighth, completed last July by Texaco, produced reports of "significant reserves" of natural gas.

"There's a lot of people got their dauber down about the Baltimore Canyou right about now," said Hal H. Bybee, a senior geologist with Continental Oil Co. in Houston. "Those of us with leases on the main (canyon geologic) structure we're pretty sick about it."

But the search goes on, for setbacks and persistance are twin companions of that cultural corner its inhabitants refer to as "the oil patch."

"We're going to give it a hell of a go, I'll tell you that," said project manager Hugh Horton, of Tenneco, Inc., who's searched for oil from the Gulf of Guayaquil to the Red Sea. "No company's going to buy a million dollar lease and never look to see what's there."

The Zapata Ugland's arrival was just the latest card in the Baltimore Canyou gamble, but one of more than routine notice. In the mechanical wonder world of offshore technology, it is one of the world's largest and most sophisticated exploration machines.

It was built four years ago on the Gulf Coast, where the science of offshore oil drilling was born with anchored barges and platforms that were towed into place and then jacked up off the ocean floor.

The Zapata Ugland, however, was one of a new generation of exploration devices designed to go further offshore and drill deeper, in ocean environments as harsh as the world can produce.

From Sabine Pass, Texas, where it was launched, it powered itself across the ocean in 21 days to the North Sea where it hunkered down and drilled its first well amid 100-knot winter winds and 70-foot waves.

If the waves were big, the rig was bigger. It was held in place by 10,000 tons of water flooded into its six silosized legs (each as tall as a 10-story building) pius ten 20-ton anchors, each winched to the rig with two-thirds of a mile of chain.

Equally as awesome as the size and capabilities of modern day offshore rigs, are the financial and corporate entanglements which produce and employ them.

The Zapata Ugland, for example, was designed in Houston and built in Texas for the Ugland Shipping Company of Grimstad, Norway, which put up its $30 million price tab.

Technically a Norwegian flag ship, it was leased under a 12-year charter to Zapata Inc., a multi-million dollar, Houston-based conglomerate which also dredges rivers, mines coal and copper, and fishes for anchovies.

Zapata, in turn, has the rig and its crew under charter (for $22,600 per day) to Tenneco whose multi-billion dollar, world-wide oil interests crowd under the same corporate umbrella that also shelters makers of auto mufflers and processors of pistachio nuts.

Tenneco, for its part, leased its nine square miles of Baltimore Canyou ocean floor in partnership with Aminoil, a devision of the same firm that was until recently known as R.J. Reynolds tobacco company.

Aminoil paid 35 percent of the $8.2 million bid for the lease and will receive 35 percent of whatever Tenneco finds.

Tenneco's actual drilling partner, however, will be Exxon, which is putting up half of Tenneco's $65,000 per day drilling cost (of which the rig rental is only a part) in return for access to the geological data produced by the well.

The rig population is similarly tangled. There are 15 marine crewmen -- Norwegians hired by Zapata from Ugland -- who handle and maintain the nondrilling aspects of the rig, including transatlantic trips when the marine crew swell to 42.

Driving a 410-foot high, one and three-quarter acre island across the ocean can have its frustrations. On the Zapata Ugland's 45-day trip here from Ireland, it encountered 80-knot headwinds and 40 foot seas which played havoc with the rig's customary 9-knot forward speed. One day it went 65 miles backwards.

Augmenting the marine crew are some 30 men in the drilling crew, plus caterers and cooks who serve up Cajun gumbo and Texas beef.

There are frilling bit salesmen and pipe suppliers, whose weres chew through clays and sandstones beneath the sea and funnel samples to the surface; purveyors of mud and ceent, whose products lubricate and line the hole as it inches downward; loggers and chemists who seach the sucked up bootom samples every few feet with computers and test tubes and monitor the rig's discharges over the side.

Supplying the rig as it works will be two $7-to- $10 million work boats, each half the size of a destroyer, and each chartered with its crew for $4,000 a day, plus a 16-passenger Westland Puma Helicopter chartered for $55,000 a month (just to be available) plus $500 for every hour it flies.

And safeguarding the massive undersea plumbing of the rig and well will be a constant on-board crew of six SCUBA divers trained to work and weld in the chill, mind-numbing darkness below 400 feet. The divers frequently work long hours with bloodstreams totally saturated with nitrogen and then spend days in a pressure chamber recovering, just an air lock away from death.

If the Zapata Ugland itself has a few unusual features, there if little unique about either its needs or its expenses or the massive logistics it will require in the Baltimore Canyon

And though its men have a lot of pride in themselves and what they do, there is little happening here to startle any frequent inhabitant of the oil patch, wherever he may be on earth.

Tenneco spokesmen estimate it will take about 100 days and $9.5 million to drill the company's well on the western edge of Square 642 in the Baltimore Canyon.

Exxon, whose chartered drill ship Glomar Pacific is working just a few miles from the Tenneco site, spent nine months and $30 million on one hole before capping and abandoning is last month.

Conoco spent two months and $4 million on the dry hole it capped last June.

Texaco paid out $10.5 million to find the Baltimore Canyon's only gas or oil discovered so far, and is testing a second well on the site.

Hugh Horton, Tenneco's burly and unflappable project manager from Quitman, Texas, worries that the zones of the gelogical layer cake in the Baltimore Canyon are far thinner than oil men like.

"Nobody but Texaco's got even a smell of oil," he says. "Not even a smell. The sands are impervious -- just like that plasterboard there."

But Tenneco's lease is just south of Texaco's in the same basic geological structure, where other hydrocarbons could yet be found.

The potential stakes are enormous. On Alaska's North Slope, where oil was discovered a decade ago, the companies who drilled there stand to realize after-tax profits of an estimated $4 billion over the next five years.

That, as the oil men say, is a lot of dinero, and it is toward that prospect they sail when, after 10 days of pre-drill maintenance and supply, the Zapata Ugland once again will put to sea from this port. CAPTION:

Picture 1, Offshore oil-drilling rig is part ship, part island. Tenneco Inc.; Picture 2, Seven oil companies are now involved in drilling operations in the Baltimore Canyon area. Christian Science Monitor