Mark Twain once cabled home from London to say that reports of his death had been greatly exaggerated. They still are. He is celebrating his 150th birthday this year, while remaining what his wife always called him: "YOUTH."

And from Broadway -- where "Big River," the musical of Huckleberry Finn, has just swept the Tony Awards -- to Hannibal, Mo., where the seven-month Sesquicentennial Celebration is in full swing, America is celebrating that birthday with him, because -- more than any other writer -- Mark Twain is our youth. He has come to represent a national childhood, unfettered, unfallen, that takes us to Disneyland to ride steamboats to the island where Tom and Huck are still playing pirates.

Twain is one of America's favorite fictions, no more real and no more dead than Tom Sawyer was when he sneaked into the choirloft and spied on his own funeral. Twain's image (the cigar, the bushy white mustache and eyebrows, the rumpled white hair and suit) is as instantly recognizable as the Statue of Liberty.

Some time ago, I took a 15,000-mile literary pilgrimage around this country. It was a hyperbolic, roving and relentlessly American scheme. I was determined to trace the sprawling map of our nation's fiction by visiting all the writers' homes, all their graves, all the settings of the books that have gone into the making of my heritage as a novelist.

The rush of this trip was very much in the Twain vein. I was on the road, I was roughing it; like Sam Clemens (most of whose books are actually travelogues, who sent Huck down the river on a raft, Tom off to Europe in a balloon and himself around the world as an Innocent Abroad), I was "lighting out for the territory."

Throughout this journey, I ran into Mark Twain everywhere. In Hartford, Conn., I joined the jammed crowds touring his Nook Farm mansion, a 19-room Gilded Age Tudor extravaganza decked out -- with balconies and porches outside and Victorian geegaws inside -- to mimic a gaudy Mississippi stern-wheeler, including a billiard table in the study, child-size bathroom fixtures in his daughter's suite, Venetian cupids on his bedposts. (What did Harriet Beecher Stowe, across the lawn, make of it all?)

In Jackass Hill, Calif., I wandered for hours on back mountain roads to find the little empty cabin where the young busted prospector Sam Clemens struck the gold of "The Jumping Frog of Calaveras County." To my surprise, I read in the worn-out guest book on a shaky old table there that 30 other pilgrims had already paid their respects earlier the same day.

In Elmira, N.Y., where Twain is buried among his wife's people, I saw the octagonal study she had built for him in the shape of a riverboat pilothouse. There are Twain plaques and memorabilia, Twain sites and settings all over the country, from Boston to San Francisco.

But his heart's home, and the home of his best-loved fiction, is a little Missouri town called Hannibal, on the western bank of the Mississippi River, a hundred miles north of St. Louis. Sam Clemens arrived there in 1839 at age 4. (His birthplace, a log cabin 30 miles southwest, is now glassed-in under a modernistic paraboloid Memorial Shrine in Mark Twain State Park.)

He left Hannibal at 18 to begin the restless vagabonding that would give birth to Mark Twain. But as Mark Twain, he would return in book after book to the town he rechristened "St. Petersburg," the town of his childhood, where the golden days "trooped by in their glory."

You can arrive in Hannibal as Sam Clemens came and went -- by boat on the Mississippi; for the last of the grand gaudy steamboats, the Delta Queen, still stops there on its "Huck Finn Trip," with calliope squealing, with passengers on deck flying kites and with "all-American officers" shouting, "Mark three! Quarter twain! Mark twain!"

My wife, daughter and I drove up from New Orleans, through Faulkner's Mississippi delta, across the Cherokee Trail of Tears and along the beautiful Great River Road, past old French fur-trading towns where river pilots like Clemens once docked their big bright vessels. Whether you come rolling in from the south on the Mississippi, or west from Mark Twain Lake, or east over Mark Twain Bridge, or north past Riverview Park where picnickers gather near the giant statue of the novelist -- Mark Twain welcomes you from all directions. From the Mark Twain Lighthouse atop the still idyllic Cardiff Hill to the bronze sculpture at its base of barefoot Tom and Huck -- when you enter Hannibal, you are stepping into a world of fiction brought to life.

Hannibal understands that it is a glorious figment of a vast imagination, and the Hannibalians have been living happily inside the fiction for years. They are proud to be the town "where Mark Twain lives again," where you can sleep at the Tom 'n Huck Motel off Mark Twain Avenue or at the Injun Joe Campground just past Huckleberry Park; where you can shop at the Becky Thatcher Bookstore or Aunt Polly's Handcrafts or the Huck Finn Shopping Center; where even the First Presbyterian Church advertises itself with the slogan, "Mark Twain Slept Here."

Hannibal lives for and through its immortal native son. Both the literary pilgrim and the vacationing tourist will find that its surrender to Twain's fiction makes it a wonderful town to visit. As Norman Rockwell said, after visiting there to research his now famous illustrations of "Tom Sawyer" and "Huckleberry Finn," the "whole town of Hannibal lives in the memories of Tom and Huck -- or, rather, of Mark Twain."

For myself, I like to feel the past through people, to shuffle my feet through history, to run my hands over relics. I like standing in Willa Cather's prairie, I like staying at Mr. Pickwick's inn. I don't make much distinction between "real" people, and fictional ones. Neither does Hannibal, and that, too, is part of its charm. They are glad to show you, with similar enthusiasm, Twain's desk and Aunt Polly's room and the yardstick used to measure the 50,000 children who auditioned for the film "Tom Sawyer." They invite you to tour the mammoth limestone cave where Jesse James in fact kept a hideout, and where "you'll have the same exciting adventures as Tom Sawyer and Becky Thatcher did many years ago."

In the center of the meticulously restored Main Street storefronts and Hill Street Historic District, a block from the river docks, is the small modest frame house where Sam Clemens merrily misspent his youth, the house that looked to the returning celebrated author "like a birdhouse." On one side is a museum, on the other Tom Sawyer's unforgettable white-washed fence.

Across the brick-cobbled street is "the home where Becky Thatcher grew up and the law office where Twain's father worked." It is not surprising that by the time she reached her eighties, Laura Hawkins, Sam Clemens' first sweetheart, was referring to herself as Becky Thatcher. (Just as Thomas Wolfe's brother Fred asked that his tombstone be engraved: "Luke of Look Homeward, Angel.") Great fictions have the power to overwhelm facts, as well as outlive them.

Down in the Mark Twain Cave our daughter Meg shivered happily beside a family, all wearing "Little House on the Prairie" sweatshirts, when the guide turned off the lights so we could feel what Tom and Becky must have felt when their last candle spluttered out. Less is said about how Injun Joe, trapped in that cave, gnawed at live bats until he starved to death.

But, then, Hannibal is not celebrating the dark Twain, who painted into his children's classics tales of violent deaths, ugly crimes and other sad and cruel evils of the species he called "the damned human race." Hannibal is celebrating Twain's and our own nostalgic dream of an Edenic childhood beside the magical river, where we are forever pals, forever young, "tranquilly and continuously happy." The town's offical motto: "We invite you to be a kid again."

We go there to believe again in our barefoot, rambling, kind and glad-hearted innocence. In Hannibal, especially in summer, especially with children, especially around the Fourth of July during National Tom Sawyer Days -- when everyone joins in raft-racing, and the jumping frog contest, and the fence-painting competition -- believing is easy.

Maybe there's something in the river air of this sleepy town that nourishes such sanguine faith: It's curious to note that the Unsinkable Molly Brown was herself a Hannibalian, and so was Ukulele Ike, who sang Jiminy Cricket's "When you wish upon a star . . . Everything your heart desires will come to you." Or as the plaque says on Twain's boyhood home, which presidents such as Harry Truman and Jimmy Carter have stopped to read: "Mark Twain's life teaches that anybody however humble his birth and surroundings may by honesty and industry accomplish great things." Not that "industry" is the first word to come to mind when we think of Tom playing Robin Hood or Huck catching catfish. It is the fun of Twain's world that Hannibal captures.

After weeks of visiting the musty homes and somber graves of other American writers, our daughter was delighted to go to Hannibal and ride the Trainland tram around town, and to ride the stern-wheeler Mark Twain that floats past the long, narrow wooded Jackson's Island, where the slave Jim hid and where Tom's Gang played hooky. From the boat you see the landscape much as it must have looked to the young Sam Clemens -- the wharves and waterfront, the cave hollow landing, the sloughs and levees, Lover's Leap and Cardiff Hill.

In town we saw the dioramas, the wax museum of Twain's characters at the Haunted House, we had an old-fashioned photograph taken, ate in an old-fashioned ice cream parlor and bought a Becky Thatcher doll. In the afternoon, we even jumped into the cool muddy river for a swim. We were all kids again. One evening, we went to Clemens Amphitheatre to see dozens of the costumed local citizenry perform their engaging outdoor drama, "Reflections of Mark Twain," in which the novelist is returned to earth by an angel to see his books acted out.

On a 140-foot stage they've built "full-scale recreations" of the riverfront buildings "just as they were," and they float a real 50-foot paddlewheeler right past the audience. I bet Twain would have loved it.

All these attractions are ordinary tourist fare in Hannibal, which is accustomed to welcoming 500,000 visitors a year. This summer they expect three-quarters of a million. And they are throwing quite a party for them, because this summer Hannibal is celebrating Mark Twain's 150th birthday -- a half-year-long fete that reaches its most festive this week, during the Fourth of July National Tom Sawyer Days.

Opening weekend was May 4, with parades, picnics, fireworks and the Navy Band. There followed dances, auctions, antique car rallies, kite days, the Northeast Missouri Shriner's Whole Hog BBQ Pork-a-rama and a rock 'n' roll festival called "Good Golly Aunt Polly."

But all summer long Hannibal will continue to host bandstand concerts, balls, street performers, puppet shows, storytellers, art exhibits, photography contests, dolls and quilt shows, folklife and crafts fairs, and, of course, more fireworks.

For small children, there will be 19th-century games, a pet-a-pet zoo and a cave maze. For teen-agers there will be rock groups from Sam and Dave to Air Supply, as well as athletic competitions such as bicycle marathons and speedboat races.

Among the hundreds of special events added this year to Hannibal's already rich tourist offerings will be music festivals for almost every imaginable taste: ragtime, Dixieland, country, big band, blues, swing, jazz and bluegrass. The St. Louis Symphony will perform the "Mark Twain Suite." There'll be military bands and a "Rollin' on the River Rock Weekend." There'll be bagpipes and bell ringers and gospel singers. Historical trains and river regattas and circuses and scout troops and university choirs will stream into town.

For the literary, Hannibal offered a Writers Conference in June. For those who imbibe, a wine festival takes place in July. For the stagestruck, an Amateur Talent Show in August. In September comes "The World's Largest Ice Cream Social," and on Nov. 30 (Twain's birthday), Hal Holbrook will appear in Hannibal to perform his virtuoso one-man show, "Mark Twain Tonight."

Who could ask for anything more at his birthday party? And what could be more fitting for the most American of writers than that the highlights of that party should take place in the heart of the country, on the banks of the Great Mississippi, on Independence Day?

As Barnum & Bailey used to say, this is a show for children of all ages. The festivities the indefatigable people of Sam Clemens' home town have scheduled for this Fourth of July week include such summery Americana as: tricycle races, a Tom and Becky contest, mud volleyball, raft racing, mule and frog jumping, bonnet and mustache judging, a parade, a cook-off and the local/state/regional and national finals of the fence-painting competition.

Could Tom Sawyer ever have thought, that bright Saturday morning, as Aunt Polly thrust the hated bucket of whitewash into his hands, that one day his town would host the national finals of a fence-painting competition?

Back in his grave in Elmira, N.Y., Mark Twain would be chuckling with pleasure -- except, of course, that he isn't there. Reports of his death have been greatly exaggerated.