The sound you just heard was the slamming of 100 million car doors. Americans are tooling off down the highway in search of sun, serenity or -- in at least a few cases -- soup.

They'll encounter a different world: a place where pigs swim, garbage is on the menu and a nuclear reactor is hot. It's no place like home.

"There's a certain wonder that you can just turn the corner on the interstate and there will be a snake farm, or the world's largest prairie dog, or the Land of Kong -- home of the world's largest man-made King Kong, with blinking red eyes," says Ken Smith, who's seen all three and about 200 similar attractions.

"It's part of America, and I think it's wonderful."

Even if you're not quite so enthusiastic about whatever is looming beyond the bend, a little advance planning is helpful. To get on the right path, here are a few clues to the unlikely and some signposts from the experts: The Pork in the Road

When you really want to see a pig get into the swim of things -- and every year, 450,000 people do -- there's only one place to go: Aquarena Springs in San Marcos, Tex.

Right in the middle of the Lost Continent of Atlantis show, a pig named Ralph takes a swine dive and dog paddles the entire length of the World's Only Submarine Theater. It only lasts about a minute, but count on it: people drive for miles to see this.

Considering Ralph is the star, it was a happy accident he became part of the show in the first place. It seems one of the mermaids had a pet pig. While playing with him, she realized he could swim, and a star was whelped.

Generally, Ralph takes to his pool like a duck to water. But sometimes, says Rich Westfall, director of marketing at Aquarena Springs, they have to give him a little encouragement.

And how do they do that?

"We push him in."

In the beginning, Ralph wasn't always Ralph. The six performing pigs all got the same stage name when the boyfriend of a mermaid went off and left her high and dry.

"So she turned around and saw the pig getting ready to swim and said, 'You know, he looks just like my ex-boyfriend Ralph,' " says Westfall. "And the name stuck. Kind of sounds like a story from 'Dallas,' doesn't it?" Get Off at Any Exit

Ken Smith, Jack Barth, Doug Kirby and Mike Wilkins are all normal-looking men between the ages of 26 and 29. They like to drive fast cars, eat fast food and quickly visit places like the Lawrence Welk Resort Village in Escondido, Calif. (site of the World's Largest Champagne Glass), and the Henry Ford Museum and Greenfield Village in Dearborn, Mich. (home of Thomas Edison's last breath, caught in a test tube).

In fact, they seem to have visited every tourist site in the lower 48, and to prove it have written Roadside America (Simon and Schuster/Fireside, $9.95).

Here's how an average day of research went for the team of Smith and Wilkins, as recounted by Smith: "We got up in Idaho at 6 in the morning, got a cup of coffee and were on our way. We tried to go to the Old-Time Fiddlers Hall of Fame, but it was closed. Then we went to the World's Only Man-Made Geyser in Soda Springs. That was good, because it was erupting. We were going to head to Yellowstone, but we saw signs that said 'Little America, 150 miles away,' so we detoured. It's the world's largest gas station -- 75 pumps, and little penguin souvenirs."

Later, they resumed their trek to Yellowstone, stopping first in Douglas, Wyo., the world capital for jackalopes -- a mixture of jackrabbits and antelopes identified in Roadside America as either an interspecies love child or the result of genetic engineering.

"We rode it and got pictures taken, and then continued north to Yellowstone, which we hated because people drive too slowly there. Then we went to Cody, Wyoming, and the Buffalo Bill Historical Center. Before we could get to Anaconda, Montana, and the world's largest smokestack, it was getting late, so we went to a motel in Billings."

After their 700-mile day, they slept the sleep of the just. "You get your own energy from it," says Smith. "You knew you could only do this once in your lifetime, and some days you felt you would only want to do it once."

His favorite? The Prehistoric Forest in Marblehead, Ohio.

"It's a bunch of dinosaurs, all of them very crudely animated," he says. "You're equipped with miniature M-16 rifles and the tour guide screams, 'Kill the monsters!' There's no educational value at all."

* If you think you'd also find such places fun, Smith offers these guidelines:

If you see a billboard advertising something at least 150 miles away, that means it's good and you should go.

Steer clear of the larger, more staid corporate parks like Disney World and Marine World. Look instead for the outlandish, like Twitty City in Hendersonville, Tenn. -- an entire attraction built around singer Conway Twitty's house.

Above all, remember that if you're on vacation, you're a tourist, and you should act like one.

"You're going some place to be entertained," says Smith. "You shouldn't have a la-di-da attitude about it. You stay for an hour and a half, have a good time, and then whoosh, you're off to your next adventure." Up and Atom Since Three Mile Island had its accident, tourism at the Middletown, Pa., nuclear power plant has soared. In the nine years prior to the mishap, a total of 150,000 visitors went through the visitor's center. In the seven years since, there have been 410,000 -- plus about 40,000 people who have actually been allowed inside the plant.

"We recognize very well that we had an accident and we're completely willing to talk about it," says GPU Nuclear Corp. spokesman Gordon Tomb. "The tours tend to demystify the plant, and that's in the interest of everybody." On sale at the visitor's center: T-shirts with the slogan "TMI Lights Up My Life" and coffee mugs in the shape of cooling towers. Tureening the Tables

"Look at the drive-in," says Ralph Collier, although it's clear he'd rather not.

"There are people munching on rubbery hot dogs and drinking liquefied mucilage with no flavor and no nutritional value at these one-armed fast food joints."

And now, regard the soup tureen.

"It harkens back to an era when people lived with much more style, and had something called table manners."

Collier is president of the Campbell Museum in Camden, N.J. If you have a thirst for soup, this could be your summer to visit its collection of 300 18th-century tureens.

One look and it's easy to see why the Germans called them "pomposity pieces." They're big, elaborate and as self-important as their original owners.

"The tureens belonged to royalty," says Collier. "They had their own factories, and they were up to their navels in porcelain after a while."

In these more democratic days, any peasant who goes to Camden can feel the same way. And they'll get something else. In the summer, visitors are offered juice; in the winter, there's soup. Campbell's, of course. Food for Thought

Like Pikes Peak or the Mojave Desert, certain questions are always lurking out there in the distance. Are we there yet? How many more days until we get home? Where's the nearest bathroom? When are we going to eat?

It's the last query that comes up most often for Jane and Michael Stern. Over the past decade and a half, they've tracked 40,000 miles a year for their guidebooks, which include Roadfood and Goodfood. And they still haven't found a decent place to eat in Montana or North Dakota.

"You go 400 miles for a decent piece of pie," says Jane.

"We drive real fast there," says Michael.

Everywhere else, they've a higher average: About one good meal for every seven that are adequate, abandoned, or worse. But the Sterns are pros, eating the tasty and rejecting the tasteless so America can digest more easily. For the more modest vacationer, the odds are even better.

"Assuming you're an amateur but not an idiot, and assuming you're willing to walk out of a place on occasion, one out of two meals will be pretty good," says Michael.

* The Sterns have well-honed gustatory radars, antennae that can sense quality barbecue or tenderloin catfish four exits away. Among their suggestions:

Do Your Homework. "Ask around. If you're going to Texas, for instance, you must know someone who's lived there, or just passed through," says Michael.

Adds Jane: "Find out what the local specialities are. In Rochester, New York, if you see 'garbage plate' on the menu, you otherwise might not know what it is, which is like going to France and not knowing what pate' is."

(Garbage plate, Michael explains, is a layered dish that starts with baked beans, then adds macaroni and cheese, potato salad, chili sauce and raw onions. "It's as good as it sounds," he says.)

Watch Out Whom You Ask. The mythology of the open road says that truckers know the best places to eat. No longer, say the Sterns.

"They used to know, when trucks weren't 50 feet long and could pull off the road," says Jane. "But now they're prisoners of the interstate."

"If we're looking for a place for breakfast," says Michael, "and we see a bunch of cop cars, that's pretty good. Unless their lights are flashing."

"Desk clerks and gas station attendants are atrocious," says Jane. "All they tend to know is the fast food places. And the worst places to find out where to eat are the state tourism offices, which are sheer boosterism."

By the time everyone not to ask is eliminated, you could starve to death.

"Ask someone at the post office," Jane says. "Mailmen often have good tips. And you can ask most ordinary people, if you phrase it right. Just ask where they eat."

Let Your Fingers Do the Stalking. Read correctly, the Yellow Pages can be the hungry traveler's greatest friend.

"A big ad is a bad sign," says Jane. "So is any ad that says 'banquet facilities for 500' or 'world-famous.' The best ads are usually small boxes, and say in simple type something like 'Mary's Cafe -- since 1929.' "

"Any place that says 'home of' . . . that's always intriguing," adds Michael. "And if a restaurant has a motto, it's got to be good."

* "The idea of having a motto is old-fashioned, corny and not very 1980s," says Jane. "It means they're somehow stuck back 30 years ago, which means the food may also be old-fashioned and homey."

Casing the Joint. "A mansard roof, the overhanging kind, is nearly a sure sign of disaster," says Jane. "But local license plates are always good. And if they're proud of their pies, they'll have them on display."

"Use your nose," says Michael. "If walk in and you're hit with a wall of grease, turn around and leave. But if you can smell hickory smoke barbecue, or baking bread . . . "

If you can smell either of those, sit down and have a good meal. You've earned it.

"I don't understand people who travel around the country and eat at McDonald's," says Michael. "They're exactly the same everywhere. If you're going to do that, why bother to leave home?