A journey beneath Monterey.

Three smokestacks rise, like motionless pistons, from the corrugated iron roof of the Hovden Cannery boiler house at the western end of Cannery Row. Abandoned 14 years ago, when the commerical sardine fishery crashed, the boiler house and the razed factory site surrounding it now abound with human activity and Monterey Bay marine life. From the rubble of Cannery Row, immortalized by Steinbeck, David Packard, co-founder of the Hewlett-Packard Company, and his wife, Lucile, built the largest aquarium in the United States.

The Monterey Bay Aquarium (MBA), which opened to the public in October, promises its visitors a heightened awareness of the dynamic and fragile world of the sea. The 1 million people expected to tour the aquarium over the next year will take an undersea journey past 83 ''habitat'' tanks exhibiting 5,000 marine plants and animals. These represent 300 species of fish, mammals, birds, and plants inhabiting the Monterey Bay.

Visitors can watch the underwater movements of the California sea otter and explore the deep reefs, sandy sea floor, and shale reefs of the open bay. They have the opportunity to observe bird and fish life in an aviary that replicates the central California shoreline, to feed cruising bat rays from the edge of an 18-inch-deep pool, and to remove sea slugs and anemones from a quiet touch-pond. They also can examine microscopic species of plants and animals in classrooms as well as a kelp forest laboratory.

The seabed here is unusual. The bay hides a trench the size of the Grand Canyon. Known as the Monterey Canyon, the 70-mile-long submarine trench plummets to a depth of over 10,000 feet, creating one of the richest, most varied marine environments on the Pacific coast. Upwelling, a process by which cold, nutrient-rich waters percolate up from the canyon depths, supplies the basic fuel needed to populate this variety of habitats.

Five marine research facilities, including Stanford University's Hopkins Marine Laboratory, have been set up along the Monterey Bay to study the marine life that flourishes here.

''The regional focus of this aquarium makes it unique among aquariums of the world,'' says Dr. Steven Webster, director of education at the Monterey Bay Aquarium. ''Most other aquariums exhibit exotic species of plants and animals. The Monterey Bay Aquarium draws its specimens only from the Monterey Bay.''

Eight years before construction began, the aquarium started as the shared vision of four Hopkins Marine Lab biologists: Robin and Nancy Burnett, Charles Baxter, and Dr. Webster. David and Lucile Packard put up the $40 million needed to make it a reality. The staff members at Hopkins and the three other marine labs that ring the bay helped design the exhibits and guide the research. The focus of both will be the ecology of Monterey Bay.

Unlike the San Francisco Bay, the coastal inlet to the north, Monterey Bay is not an estuary. Fresh water from the Sacramento River flows into San Francisco Bay, diluting the saline waters of the Pacific and limiting the diversity of bay marine life. The Monterey Bay, whose Elkhorn Slough silted up years ago, resembles the open sea. Twenty-six species of marine mammals, including the migratory humpback, blue, and gray whales cruise the bay.

''The Monterey Bay is the southern boundary of a biological range that extends from Monterey to Japan and the northern boundary of a biological range that extends from Monterey to Baja California,'' Dr. Webster says. ''The bay sits in the middle of a third range for California species of plants and animals.

''Close to 80 percent of all species of fish, plants, birds, and mammals found between Baja California and Anchorage, Alaska, live in the Monterey Bay,'' he adds. The geology of the bay provides ''a diversity of habitats for these critters; upwelling provides the basic fuel for the bay's complex food chains; and three ocean currents get them here.''

Sixty-five species of rockfish live in the Monterey Bay. One species of rockfish lives off the Massachusetts' Cape Cod.

''Fortunately, this aquarium is not located on a busy, polluted harbor,'' Webster continues. ''Unlike so many aquariums, the Monterey Bay Aquarium can pump raw seawater into its exhibit tanks. This enables us to maintain whole biological communities, from the single-celled, submicroscopic organism to the largest fish in our tanks. The wharf pilings in the Monterey Bay tank are covered with filter feeders, because the bay water that we draw into the aquarium at 2,000 gallons per minute contains the microorganisms that these barnacles need to survive. We have been so successful at building complete biological communities,'' he adds, ''that the Monterey Bay tank, a 90-foot-long, hourglass-shaped tank, holds a deep reef, a sandy sea floor, a shale reef, and a wharf community. To sample as much of the Monterey Bay as is contained in this exhibit, you would have to swim at least three miles in the bay.''

One tank here displays an entire kelp forest community. Two-and-a-half million pounds of water slosh across this 66-foot-long by 28-foot-tall tank, the tallest in the United States. The exhibit replicates an authentic environment for a complex undersea community with the help of a five-foot fiber-glass plunger that pivots from an eight-foot derrick. This surge machine creates a wall of water that rocks from one end of the tank to the other.

''No aquarium or research facility has ever grown giant kelp in a natural setting outside the wild,'' Dr. Webster says. ''We have engineered the kelp forest tank to make that happen. Kelp absorbs its life-giving nutrients from the gentle movement of seawater across its notched blades. The kelp in this tank grow at a rate of 4 inches a day.''