Tiny shrimp carpet the depths in unimagined profusion; steep, rocky cliffs rise abruptly from a smooth plain; fish carve and sculpt caves so diligently that the bottom is actually being changed; sunken valleys are so thickly covered with algae that they look like "alpine meadows."

This picture of abundance, to the surprise of scientists who explored the area recently, is Lake Superior, North America's mightiest body of fresh water.

The explorers say their pioneering voyage in the minisubmarine Sea-Link II hundreds of feet below the surface revealed important new information about the largest of the five Great Lakes.

"It has given us an important feeling for the first time of what the lake is really all about," said Prof. David Long, a principal investigator on the expedition. "It's calm and quiet, far more than we had anticipated, and that changes the perspective about Superior."

Scientists at midwestern universities and institutes said data being analyzed from their monthlong expedition may lead to new understanding of major environmental issues facing the United States and Canada.

The scientist learned so much about fish, aquatic plants, geology, chemistry and shipwrecks on the bottom that a second series of extended minisubmarine probes is planned for next year.

Superior's depths have been poked and prodded from the surface for almost two centuries by people along its shores. In the last two decades, extensive information has been gathered by divers using self-contained underwater breathing equipment.

But until the arrival there three months ago of Sea-Link II and its mother ship, the research vessel Seward Johnson, no submersible had been capable of extended, deep-water investigation of the lake.

Since the inland lakes became seriously polluted in the postwar years, U.S. and Canadian scientists have sought to learn the exact extent of the pollution and the lakes' ecology. They sampled water, bottom layers, temperature and wind and water currents.

Because Sea-Link II can dive to great depths for as long as four hours, it is perfectly suited to the task. Self-propelled, it has a large, spherical glass cabin and looks like a fish bowl surrounded by plumbing.

The sub and mother ship tracked slowly across more than 500 linear miles of Superior, beginning in late July and ending about a month ago. The voyage started in Sault Ste. Marie, Mich., on the lake's southeastern corner, roamed west along the southern shores, north to remote waters off Isle Royale and returned to Sault Ste. Marie.

More than two dozen scientists made dives in Sea-Link II. Among the most important of their preliminary findings, they reported, is that the lake's waters are far more abundant with fish and aquatic life than had been indicated by surface surveys.

Dr. Jerry Kaster, a biological sciences professor at the University of Wisconsin -- Milwaukee, described finding the lake bed thick with tiny crustaceans known as opossum shrimp, a major food source for fish. During the day, these shrimp lie on the bottom; at night, they approach the surface to feed.

No light is apparent 900 feet below the surface, day or night. When the sub's spotlights pierced this blackness, Kaster said, he was astonished because the lake floor sparkled with millions of tiny dots of light.

"You could see the shrimps' tiny eyeballs everywhere. The water lit up with the reflection of their eyes. I never expected it to be like that. The lake bottom is an order of magnitude richer than we have realized," he said.

Kaster and several colleagues are analyzing shrimp collected to determine pollutants in them. "Are they transporting contaminants and nutrients from the bottom sediments into the water column that stretches up to the surface? This is extremely important to understand, because opossum shrimp are heavily relied upon by the lake's fish for food," he said.

Meanwhile, Tom Edsall of the U.S. Fish and Wildlife Service in Ann Arbor, Mich., is analyzing data collected by Sea-Link II near spawning grounds of the lake trout, a major game fish that suffered catastrophic declines after an invasion of lamprey eels. These predators attached themselves to ships navigating the extensive canal system linking the Great Lakes to the ocean.

"We went looking for the spawning grounds of surviving native lake trout to find out what allows them to survive when so many others have not," Edsall said. "About 100 million lake trout have been stocked in the Great Lakes, but the success rate of survival has been very low.

"We found the historic spawning grounds to be very clean, no visible pollution, with much oxygen in the water. The sub gave us a fish-eye view of the grounds, and we found them first-rate. Superior would rate a 10 on a water quality scale of 1 to 10. Lake Erie would rate a zero in some areas."

Such information from the underwater probe of the spawning grounds may provide clues to improving the lake trouts' chance of survival in years to come.

William Cooper of Michigan State University, the expedition coordinator, described surprise at discovering how burbots, large lake fish, carve caves and depressions in the soft, glacial clay comprising much of Superior's bottom.

"They may have enough impact to actually alter the configuration of the lake bottom," he said.

He said the expedition found a large mass of living organisms "in the depths, very low growth rates, and so the reproduction rates are probably fairly slow. This means that the lake bottom can't take a lot of harvesting because the populations down there won't be replaced quickly."