Down in a low-ceilinged basement of the Smithsonian's National Museum of Natural History, female blue crabs work their way from fresh water to salt water to hatch their young each spring, just as they do in the wild.

Oysters grow in brackish water, snails cling to slender stalks of marsh grass and fish swim in the shallows of water tanks that have been assembled to form the closest artificial re-creation ever made of the Chesapeake Bay.

It's a good show and one that deserves an audience, says Walter H. Adey, the creator of the bay-in-the-basement, a 5-year-old experimental system that has never been open to the public.

"How can you really understand an ecosystem?" he said. "I think people can, but they have to be shown how. They have to see it and walk through it."

Now Adey, who has been the Smithsonian Institution's marine systems director for 14 years, is preparing a detailed proposal for a permanent Chesapeake Bay exhibit that, if financed, could open by 1992. The Ethyl Corp., of Richmond, has given Adey a $300,000 grant to research the project and will decide later this year whether to finance the $1 million-plus exhibit.

Adey is an enthusiastic promoter and leading practitioner of an emerging science that he calls synthetic ecology. During his studies of ocean reefs in the 1970s, he tried to duplicate the complex aquatic system in a closed model. He calls such models "mesocosms" -- bigger, more elaborate versions of microcosms used in laboratory research.

"It turned out to be simpler than I thought," he said. Models he has built since then include a coral reef exhibit for the Australian park authority and a model of the Florida Everglades, also closed to the public, that occupies a greenhouse at the U.S. Soldiers' and Airmen's Home in Northwest Washington.

His most ambitious and notable work is the construction of a giant marsh, a 2-million-gallon ocean and underwater reef, one of the seven interconnected environments for Biosphere 2, the scientific venture in a four-acre glass enclosure north of Tucson. Biosphere 2, with its thousands of species of plants and animals, will become home later this year to eight scientists, who will work and live in the five-story enclosure for an uninterrupted two years.

Adey's work on Biosphere 2 depended heavily on the success of his miniature Chesapeake Bay in the museum here, his first attempt to model an estuary.

Estuaries contain a soup of ingredients that form shifting combinations from upstream to the bay's mouth. Adey concocted recipes of different water salinities and temperatures, nutrients and light to duplicate the bay's shift in character throughout its 200-mile length. He even developed algae scrubbers, sheets of mesh coated with seaweed and scum that could remove excess nutrients from the water and produce oxygen. And he made it so that his bay would empty into an "ocean," an assortment of boxy equipment that traps sediments and pumps cooled salt water back into the bay.

Then he populated the 2,500-gallon tanks with several hundred species of plants and animals that breed, die and coexist largely on their own. Adey and his staff play the role of top predator, and each spring they pluck out the biggest blue crabs to keep them from overwhelming the system and, incidentally, to have a nice meal.

The elaborate artifice has created a healthy ecosystem that offers a glimpse of how the real Chesapeake Bay functioned before centuries of pollution.

Models such as his, Adey said, could stand in for the bay in experiments tracing how pollutants weaken the ecosystem.

However, there are doubters in the scientific community who question the accuracy and control of such models. But Adey said he intends to pursue such research in addition to building the public bay exhibit.

He thinks that the world's chaotic diversity can be imitated and that it is worth doing. But then, this is a man who cuts his lawn in patterns to encourage its growth as a meadow, and whose artistry includes collecting mud and bugs and water and shaping them into a living community.

"It's just wonderful to make things happen," he said. "You can work with your environment. You don't have to manage everything."