Once again Chessie has reared its head. Well, its alleged head. Or something.

There is hardly a consensus about what it was that went humping past the video camera that Robert Frew, a 40-year-old computer salesman, aimed excitedly at the calm water of the Chesapeake Bay off his home on Kent Island. Something appeared three times at the surface shortly after 7 p.m., on May 31, showing more of itself each time.

It moved up and down. It was dark colored, upward of 40 feet long and thick as a human thigh. There were children 100 feet away in the water who never knew it passed. People from the shore screamed. Karen Frew said its head looked like a football.

Was it Chessie, the legendary sea serpent of the Chesapeake, christened for Nessie, the monster who sublets Loch Ness? Or, as one National Geographic photographer who viewed the videotape speculated, "four kids in a plastic bag?"

Whatever, the sighting of a large thingamajig snaking up the Chesapeake Bay has stirred more than the normal turmoil that accompanies such reports of Unidentified Swimming Objects, thanks to the three-minute color videotape.

A Baltimore-based group called the Enigma Project that investigates "paranormal phenomena" claims to have authenticated the "Frew Tape," as they call the footage, but has not drawn any conclusions about "the Frew Creature," as they call its star. Experts are being summoned for a mid-August meeting of scientific minds.

Possibilities being explored include the Mutant Eel Theory, the Giant Anaconda That Stowed Away in a Banana Boat Theory and some joking references to the effects of the Calvert Cliffs nuclear power plant on small water snakes.

Meanwhile, the creature that caused this uproar is at large, swimming where it has for at least a half-century: somewhere between fact and fantasy.

Myth or not, the elusive Chessie doesn't want for company in Maryland's bestiary of monsters. In the state's waters and woods and swamps lurk such storybook creatures as the Snallygaster, the Sykesville Monster and the Goatman of Prince George's County.

The Goatman was reputed to be a researcher at the National Agricultural Research Center who quit his job, developed a thick layer of body hair, and began attacking cars with an ax.

The Jersey Devil, however, is far and away the most famous land-based monster on the Eastern Seaboard.

"The Jersey Devil had wings and could talk," said Barry Pearson, a folklore expert at the University of Maryland. "He would fly around yelling things like 'Balance the budget.' " But the Jersey Devil has stayed close to the Pine Barrens in New Jersey, his haunt since the 18th century, even though he seems ideally suited for a hedgerow on Capitol Hill.

Realists, dripping with skepticism, dismiss Chessie as a figment of impressionable minds. How, they wonder, can scores of watermen spend their lives on the bay and never see a 40-foot serpent that moves up and down and has a football for a head?

Monsters often have proved to be something other than what they seemed. The Cow Woman of Charles County turned out to be a male hunchback. The noisy "Many-Humped Monster" that was spied once a year thrashing the waters of Lake Meru in Kenya, proved to be a female crocodile in heat being pursued by a string of bellowing suitors.

But when it comes to close encounters, Chessie witnesses are adamant. There were sightings in 1978 around the lower Potomac, and again in 1980. Helen Jones, of North Beach, Md., for example: "It was two years ago. I was out crabbing. This great big thing come up out of the water. I could have reached out and touched it with my hand. It was about as round as a watermelon, more brownish than gray and had white spots on the hump. I told Officer Tesh, 'Jack, you just missed it.' He says, 'Well I'll be doggone.' I got a big tickle when I saw it on TV. I could have told you that was in here years ago."

Given the breadth and depth of the sea, reports of ocean-dwelling monsters are more credible than sightings of land creatures. And hoaxes are harder to pull off. Roy Mackal, a professor at the University of Chicago, author of "Searching for Hidden Animals," and a founder of the International Society of Crypto-Zoology, believes unreservedly that sea serpents of Chessie's ilk exist.

"I am convinced on the basis of evidence that we are dealing with real animals that are related to primitive whales," he said. "These creatures have been sighted along the Atlantic Coast and off British Columbia for several hundred years. There were thousands of sightings made in Gloucester harbor in the 1800s. Typically, these creatures are found in estuarine environments, flex up and down, not side to side, and are fish-feeders that follow fish runs."

As T.S. Eliot noted, monsters fare well in the press--certainly better than poets. The better to liven up dead weekends, H.L. Mencken invented a wild man-beast that he said had been spotted in Baltimore vicinities. Posses were formed and mothers kept their children inside.

Thus, it's a bit ironic that Karen and Robert Frew couldn't get anyone interested in their videotape for a month or so--until, really, the summer doldrums set in, and a TV reporter from Baltimore who specializes in droll features stopped by to have a look. The Baltimore Sun got interested and, Sunday, the dam broke.

Early this week, the Frews were unable to park in their driveway. Their phone has been bombarded with some 150 calls from television and radio stations and by newspapers from Los Angeles to London.

Smithsonian zoologist George Zug, who had agreed to review the videotape in hopes of pinpointing the nature of the Frew Creature, nearly was scared off by what he called "the media circus." Zug, chairman of the museum's department of vetebrate zoology, has come around and will consider what Robert Lazzara of the Enigma Project calls, "the best evidence we have that Chessie exists."

Until a specimen turns up, however, Chessie will remain a wishful postulate and, as such, a creature that travels in the imagination rather than the scientific mainstream, a source of excitement and dread, and a resonant mystery for the few who say they have glimpsed it.

"No one felt scared," said Karen Frew. "We were yelling: 'Geeze! Look at that thing! My God! ' We followed it until it went out of sight and then we jumped in the van and headed north, thinking it might be going into the Chester River. I want to find out what the heck it was. I want it classified. I want somebody to put a label on it."