QUINTANA ROO. Not your everyday household word, although it deserves to be.

Cancun sounds more familiar. Indeed, this well-manicured, high-priced and, most recently, overbooked resort is in Quintana Roo, one of the three states that make up Mexico's Yucatan Peninsula. (The other two are Yucatan and Campeche.)

But unless creature comforts are your uppermost vacation requirement or you yearn for Miami Beach of the 1950's, you might as well forget Cancun, which helps keep tourism second only to oil as Mexico's chief industry. Quintana Roo has something much better to offer.

Quintana Roo became a state in 1974. Even now its beaten tracks arenot very beaten, and there is everywhere a sense of isolation, of jungle, of ancient mysteries and of Mayan history. We became hooked on Mexico's ancient mysteries on a trip to Oaxaca about a year ago, and, when we decided to return to Mexico the Mayan mystique drew us to the Yucata'n. There is almost no other way to do it except by car--with a local driver or without, in a group or alone. There are very few highways, so wrong turns are few. As long as you fill up on gasoline wherever you see the PEMEX (Petrole'os Mexicanos) sign, you'll be fine.

We started at the lovely colonial city of Me'rida, the capital of the state of Yucata'n, and drove from there to Chiche'n Itza', then to Isla Mujeres via the mostly unreclaimed city of Coba' in its daunting jungle, later on to Tulum on the coast, then to sparkling Xel Ha and finally to the real treasure--not for seeing as for living in--Playa del Carmen.

Quintana Roo's Caribbean coastline from the ancient Mayan walled coastal city of Tulum north to Puerto Juarez, the ferry point for Isla Mujeres, has been slow to prove itself as resort fodder. For one thing, between the coastal (two-lane) highway and the white sand beaches there is genuine jungle. In fact, Quintana Roo is still largely jungle. Oh, to be sure, it's not the canopied rain-forest-y kind of jungle to be found 1,000 miles or so to the south. Most trees, except for palms, are pretty scrubby. And of course there are the fields of sisal or henequen that provide the famous Yucatecan rope and hemp products. Nevertheless, there are parts of the drive from Chiche'n Itza' to Tulum, the southern end of the cross-jungle road, that leave one uneasily convinced that should the rented car break down, car and passengers alike would be swallowed up almost instantly by the fecund looking vines whose creeping tendrils are already reaching onto the roadway.

Indeed, legend (and some archeological evidence) has it that Chiche'n Itza' was connected by raised stone causeways to the Mayan coastal cities during the height of that civilization that lasted for perhaps 1,500 years or so beginning as early, some say, as 900 or 1000 BC. If so, the jungle has long since reclaimed them. One hopes today's blacktop fares better.

In mid-December the highway was deserted--except for occasional groups of men carrying rifles over their shoulders, crossing the road in search of what we couldn't tell. Not tourists, we hoped. (In fact, we learned later, they were probably out to find dinner.) Friends who recently retraced our steps report that, although still deserted, the highway was being widened to four lanes.

One reason it may seem so deserted is that the turnoff from the main highway at a town named Nuevo Xcam (pronounced Shkam) isn't marked--or wasn't when we were there. We happened to see a faded, painted ad for Club Med's Villa Arqueolo'gica at Coba', halfway, or a bit more, to Tulum from our starting point at Chiche'n Itza'.

And indeed, until we got to Coba' we really weren't sure we were on the right road, although, as we kept telling each other, there probably wasn't another road around to be the wrong one . . .

Coba' is a city not yet reclaimed from the jungle. Its lichen-covered structures, including a huge pyramid that defies climbing, loom bleak and black out of the wildest jungle we saw on the peninsula. In contrast, a few hundred yards away is the Club Med villa. We stopped for only about an hour, but it seemed like a prime spot to spend days. Coba''s treasures are unmarked and ripe for discovery. And when the jungle (and its humidity and bugginess) gets too much, a quick swim at the villa pool seems a likely antidote. (The five Club Med villas in Mexico--owned by the Mexican government but managed by Club Med--each sport libraries full of books and slides on the various MesoAmerican civilizations and their ruins, everything in Spanish, French and English, gourmet dining rooms, swimming pools and single, double and triple rooms.)

The Yucata'n peninsula thrusts like a thumb from the Mexican mainland, separating the Gulf of Mexico from the Caribbean. The islands off the Caribbean coast like Cozumel and Cancun (whose multi-lane causeway connection now makes it something less than a genuine island) and more recently Isla Mujeres, have traditionally been the greatest tourist draws on the Caribbean side of the peninsula. But the mainland coastline has been woefully and undeservedly neglected.

Now it is slowly being discovered and small developments are springing up along its clean, white, sandy beaches. The Caribbean may not be as quiet as the Gulf waters, but it is gentle enough, most of the time, although occasionally and perilously capricious, and the coast boasts its own share of mysterious Mayan ruins. Beach fades into jungle and the highway splits through it about half-a-mile (or less) beyond. Across the road, more jungle. A sudden but severe rainstorm whips at the car, at the vines, at the trees and when it is gone, a wealth of wildlife, as upset as we, are momentarily driven out of hiding.

An enormous toucan (right out of the pages of the bird book) flaps his wings and settles insouciently onto a tree branch, clearly for our benefit. A flash of brilliant sapphire is in and out of sight before it can be identified. A tejon darts across the road in front of our car, and our instant suspicion that we had the explanation for the ubiquitous rifles was later confirmed. The tejon looks like a cross between a possum and an oversized fawn-colored raccoon. It is, we are informed later, considered quite a delicacy. There are all sorts of delicacies in the jungle, we are told, for those who care to hunt them, along with poisonous snakes, insects beyond enumeration, giant spiders--even, one book suggested, alligators. Some things have been hunted virtually out of existence, like the jaguar--in Mayan, balam--sacred to most of the MesoAmerican civilizations. Its visage is found on virtually every Mayan structure at every major and minor ruin site. The red jaguar awaits the intrepid climber at the top of the slippery stone stairs in the pyramid at Chiche'n Itza'. (No, we didn't make it.) Some suspect that the ferocity of this speedy carnivore helped wipe out hordes of refugee Mayans as they fled into the jungle to escape invading Toltecs. Now balam itself is close to extinction and protected by law.

For a couple of days we stayed at Playa del Carmen, until recently very little more than a quaint fishing village and the site of the pier for the ferry to and from the island of Cozumel.

Now, however, there is a charming, comfortable beach-side hotel: Balam Ha (meaning in Mayan, waters of the jaguar) managed by a Texas couple, Tom and Melissa Short. Connected to the project is a group of villas and condos in various stages of completion along about five miles of beach. The project uncovered a group of minor Mayan ruins and these have been cleared of jungle and made accessible--night lighting gives them an eerie ghostly appearance. A fine restaurant is connected to the hotel, each room of which faces the Caribbean. The hotel is owned by the Barbachano family which controls virtually all the top-rated, nongovernment owned hotels in the Yucata'n.

Playa del Carmen is a perfect takeoff point to do the whole coast. On a nice day, simply drive (south to Tulum or north to Puerto Juarez) and take every road that leads toward the ocean.

You will "discover" such treasures as Xel Ha (pronounced Shell-ha) which means in Mayan "where the waters are born." It is a remarkable series of protected pools or ponds or small lagoons filled with an infinite variety of tropical fish--blue and yellow tangs, sergeant-majors, clownfish and, well, we didn't have a fish book along. The water is so clear you don't have to get in, although snorkeling is considered sensational. The day we were there it rained, so we made do with gazing from above.

We did, however, snorkel at El Garrafon beach on Isla Mujeres, hailed by many guidebooks and Mexican experts as the prime snorkeling on the continent. Those who stay on the island are urged to go early, before the boats arrive from Cancun . . . Indeed, the seductive beauty of the underwater scene can be perilous to the unwary. Caught in a riptide, we suddenly found ourselves on the wrong end of Caribbean caprice. Trying unsuccessfully to swim back the way we came, we got bashed around on some viciously sharp and poisonous coral. One of us stepped on a sea urchin, apparently a not uncommon occurrence. (You tweeze out the most irritating needles and let the rest be either expelled or absorbed. Sometimes it takes weeks. The wounds from the sharp coral took three months to fully heal. We learned later that snorkel and scuba accidents of a more serious nature occur regularly, but do not become publicly known. Even strong swimmers should pay as much attention to the currents as they do to the tangs.)

But back to the mainland coast. Another primitive-looking road off the highway leads to Akumal. Once the headquarters of a posh Mexican explorers club, it is now a small but lovely resort with hotel, restaurants, rental apartments and convenience stores.

But the main treasure of the coast is Tulum. This ancient Mayan walled city was still occupied when the Spaniards arrived, but was inexplicably abandoned soon after. In addition to its own treasures, its rocky coast boasts one breathstopping view after another. There are as many theories about the role of the city and its architecture and its mysterious doom as there are guides willing to take tourists through it. (There is even one guide who relates its construction and its frescoes to the Book of Mormon.)

Then there is Hazel Davila, a 77-year-old American woman who has spent 37 years in Mexico. She has, she says, graduate degrees in Mayan history and archeology and runs a tour-guide service with her husband. (She also has, she will tell you between archeological statistics, 32 grandchildren.) She uses Cancun or Me'rida as her headquarters and still takes groups through Tulum. She and her husband arrange larger tours of the Mayan wonders. She tells you eagerly that she recently "toured" some of the stars of the popular soap opera General Hospital. She has her own elaborate theories about Tulum, its astronomical wonders and why any given building was constructed how and where it was, and she is openly contemptuous of all others. Its million inhabitants, she says, fled into the jungle when the Toltecs were coming and fell victim to poisonous snakes, poisonous insects, carnivores and jungle diseases. It does not seem illogical. But then, in that setting, virtually nothing does . . .