Turn left off Main Street - you can do that legally on a snowmobile in this town - and you run into the gates of Yellowstone National Park.

Make a right turn and you can ride for days, even weeks, without ever stopping for a gate or anything else.A left turn takes you to geysers and paint pots and grazing buffalo. A right turn takes you to a vast wind-carved wonderland of snow and trees that is so big it takes three states to hold it.

Turn left to see the wonders of nature; turn right to ride snowmobiles.

More and more often, people are coming to ride outside the park, says Clyde Seely, owners of Yellowstone Tour and Travel.

"Yellowstone is nice. People like to spend a day inside the park. But, after that they want to go riding. Inside the park you're confined to the trails. Outside the park you can ride anywhere. There's so much to see and so many places to go, you never run out of places to ride outside the park."

Among the three neighboring states - Montana, Idaho and Wyoming - there are more than 1,200 miles of groomed trails and who knows how much ungroomed area.

So much so, in fact, two men recently broke a record they held for the most miles ridden in a 24-hour period. In 1996, Randy Gravatt and Joe Williams of Island Park, Idaho, each rode 912 miles. This past January, each rode 1,289.7 miles between 2 p.m. on a Tuesday and 2 p.m. on Wednesday.

One significant fact to come out of this ride was that a significant part could be done on ungroomed trails.

The hub of this snowmobilers' haven is West Yellowstone. It is here riders find food, lodging, fuel and snowmobile parts. It is here, too, they find the trailhead to all the surrounding areas.

Inside the park and on access routes, groomers constantly groom more than 200 miles of scenic trail.

Outside the park, it becomes a group effort. West Yellowstone, for example, grooms nightly during the winter, especially such popular riding areas as Two Tops, areas along the Continental Divide and Hebgen Lake.

Montana grooms trails to the north.

To the west, Idaho connects with more than 640 miles of groomed trails. Groomers try to smooth out the trails at least once and in most cases twice a week.

Are there enough riders to justify the work?

"Oh yes," says Chuck Wells, trails program director for Idaho's State Parks and Recreation.

"We've always had a lot of use of the trails, but in the past five years we've experienced about a 10 percent growth annually. A few years ago, the U.S. Forest Service conducted a study and reported around 1,500 snowmobilers a day were coming out of West Yellowstone and hooking up to the trails in Idaho."

Spot checks show that numerous riders are coming from the East and Midwest. Those people who are used to riding on flats and tree-lined trails in the eastern parts of the country are taken by the natural beauty of the region.

"They haven't seen anything like this . . . the mountains, the trees, the powder snow. They can't seem to get in enough riding and so they come back. Sometimes they'll go back into the park and sometimes they won't," said Seely.

The most popular ride remains the Two Tops area. Each year, heavy snow covers the trees, then high winds come in to sculpt the snow into a thousand formations. Some are obvious, like faces and animals, while others grip the imagination.

Another popular activity is to ride a trail to a destination, either to stay overnight or simply to have lunch or dinner, then return over another trail. The spider web of groomed trails makes it possible to take any number of routes and end up at the same location.

Seely was one of the first to see the potential of snowmobiling both inside and outside the park. Back in 1971, he opened a snowmobile tour operation from his Three Bears Lodge with 15 machines. It was, he recalled, a gamble. No one put Yellowstone and snowmobiling together. Yellowstone was a summer attraction.

The first years there was one restaurant open, one gift shop and a few motels. Today, West Yellowstone is a city of activities, complete with theaters, a bear and wolf sanctuary and fast-food stops.

"We decided to stay open for one winter and see what happened," he recalled. "What we found was that people loved the area and the opportunity to ride into the park in the winter. As more and more people started to come here in the winter, the demand for more riding increased.

"We started off with one groomer. Now we're running four groomers each night."

Two years ago Seely opened the town's largest facility, the West Yellowstone Conference Hotel.

What he's found is that a number of companies would sooner forgo business conferences in sunshine states like Florida and California for the opportunity to spend time in a winter wonderland snowmobiling.