Following an anchor rope straight down into the cold, green Pacific Ocean, Bob Schmieder swam toward his personal El Dorado in the fall of 1978.

Schmieder, then a Bay Area research physicist and recreational scuba diver, had plotted every detail of his dive on Cordell Bank, an underwater mountain wrapped in mystery, not gold.

Strobe lights were fixed every 10 feet to the rope, enabling Schmieder, 37, and his fellow divers to keep it in sight during their 120-foot descent.

A diver who drifted away from the line could easily become lost and confused — especially under the intoxicating grip of nitrogen narcosis — and perish in the 50-degree water about 25 miles offshore from Bodega Bay.

About 70 feet down, Schmieder encountered a dark mass that momentarily crushed his spirit as he mistook it for a featureless ocean bottom.

It was instead a dense school of fish that parted as Schmieder flippered through it.

"It was just like opening a curtain," he said, recalling a scene still fresh in his mind 35 years later. "The whole view exploded in front of us."

Schmieder, now 71 and retired, had descended right on target, next to a 200-foot-wide pinnacle at the south end of Cordell Bank, a huge chunk of granite resting at the edge of the continental shelf, where the seafloor abruptly plunges from about 400 feet to the 6,000-foot abyss.

The date was Oct. 20, 1978, coincidentally 125 years to the day since Cordell Bank was discovered, accidentally, by a surveyor named George Davidson in 1853. The shoal, marked on charts, was well known to mariners but to Schmieder's knowledge had never been seen by the human eye.

Corals, sponges, anemone and sea stars blanketed the rocky outcrops, with swarms of rockfish circulating overhead. Natural hues of red, yellow, green, blue, purple and white gleamed in water of tropical clarity, with about 80 feet of visibility.

"It was like being on a mountaintop overlooking a valley," Schmieder said in a telephone interview from his home in Walnut Creek, where he has lived since 1969.

Schmieder is convinced he was the first person to see Cordell Bank, asserting that even by today's standards and equipment a deep dive that far offshore is for experts only.

In all, Schmieder made about 15 descents to Cordell Bank, including a subsequent expedition in 1979 and several more through the mid-1980s. He wrote a 100-page book, "Ecology of an Underwater Island," published in 1991, and was onboard but not in the water for Jean-Michel Cousteau's video dives in 2006 and with a team of government divers and scientists who made the only other documented descent to Cordell Bank in 2010.

The dive is feasible only in September and October, when the water temperature and current are most moderate, but that also is the peak of the great white shark population at the nearby Farallon Islands.

Cordell Bank, protected by a marine sanctuary since 1989, also has been explored by a two-person submarine and unmanned underwater vehicles.

During the last ice age, about 20,000 years ago, when the sea level was about 360 feet lower than it is today, Cordell Bank was on dry land and may have been visited by humans. It's part of a massive chunk of bedrock — including Bodega Head, the Farallon Islands and the Point Reyes Peninsula — that sheared off from the southern Sierra Nevada about 30 million years ago and slid north along the San Andreas fault.

In modern times, the bank lay unknown until the Gold Rush prompted a need to survey the coast for maritime safety.

In October, 1853, Davidson was lost in fog near Point Reyes when he dropped a weighted line overboard in hopes of determining his position. To his surprise, he measured 180 feet, half the depth he expected. Davidson accurately guessed he had discovered a bank, but the Civil War precluded further exploration.

In 1869, Davidson assigned Edward Cordell, who had discovered a bank outside Boston harbor, to conduct a thorough survey of the continental shelf near Point Reyes. Cordell located the bank by the proliferation of birds and mammals and green water, both signs of biological richness.

Cordell's untimely death — he fell and struck his head at the corner of Kearney and Pine streets in San Francisco six months after discovering the bank — caused his name to be attached to the underwater mountain.

Bob Schmieder's diving days are over, but the descent in 1978 remains fresh. "My head is still filled with very clear memories of what I saw," he said.

(You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com.)