FEW outsiders make their way the 165 rugged kilometres north from Hanoi to Ha Long Bay. ''A fence of mighty stone pillars, each one separate, starting sheer up out of the sea to the height of cathedral towers'' is how Crosble Garstin, an explorer, early this century described this unique aquatic terrain. ''Islands take on human profiles'' he gushed, ''others look like crouching frogs, sugar loaves and ships under sail. They are prehistoric monoliths and church spires.'' There are an estimated 3,000 islets, pinnacle mountains and jutting limestone and dolomite rock formations in Ha Long Bay, many far more impressive than those off the coast of Thailand. Long a part of Vietnamese mythology, this vast field of bizarre monoliths - comprised of organic sea and plant matter compressed by water and gnawed by miniscule tide-level molluscs over thousands of years - features prominently in the tales of battle that are so much a part of Vietnamese history. In the 13th century, General Tran Hung Dao hid in these caves the stakes he would plant in the bed of the Bach Dang River to cut off the retreat of the Mongol Invaders. Vietnamese naval ships also took advantage of the large, hidden caves - some capable of holding up to three thousand people - to lie in ambush for Chinese warships. There's every chance some of the battle galleons are still there, complete with a full complement of skeletons. So plentiful are the caverns, grottos and tunnels that they are still being discovered. The familiar ones have grand names and fanciful legends - the Grotto of Wonders, Father and Son Rock, the Fighting Cocks, Customs House Cave, Surprise Grotto, the Unicorn, and the winding, three-kilometre-long Hang Hanh Tunnel. Many of the islets, covered in stunted and twisted vegetation, have tiny, perfectly-formed beaches; others have openings so high they can only be reached by pelicans and sea swallows. The yellow-haired monkeys on one particular island provide a valuablevaccine, which is used internationally. The Maiden Grotto took its name from a complicated legend involving Nang He, the beautiful daughter of a fisherman who was starved to death in a grotto by the order of a rich boat owner whose bed she refused to share. At her burial site, a rock was later discovered in her shape (which was, by all reports, rather pleasing). Close to the shore of Ha Long Bay stands the Island of Poems, rising 350 metres from the sea, where, centuries ago, poets and scholars spent their days in meditation and quiet discussion. Ha Long Bay has, of course, an attendant legend to explain away its splendour. Ha Long means ''where the dragon descends into the sea'' and ancient tales abound of fisherman sighting dragons - symbols of great power - in the area. Ha Long Bay became a popular retreat during the French colonial era a century ago. Officers, senior merchants and titled families maintained villas and retreated there from the tropical south, partaking of the herbal medicines made from the yellow aniseed common to the area. Even today, the remnants of the French era still take their holidays in the many hotels along the Bai Chay Road, which runs parallel to the main beach across from the bay. In the hotel dining rooms the animated family conversations are just as likely to be in French as Vietnamese. Of Ha Long Bay's five main hotels, the most impressive is the Ha Long Bay Hotel, a converted colonial hospital, complete with huge rooms, spacious balconies, window shutters, trundling ceiling fans and a mood unchanged since the turn of the century. You can travel by road, or there is a ferry from the port city of Haiphong to Cat Ba Island, where you can transfer to a bay cruise, but most visitors are taken directly to their Ha Long Bay hotel for the pause that refreshes before being taken out into the bay for excursions.