Geyser gazers who watch over Yellowstone’s natural wonders consider it a labor of love.

On the day before the Park Service stopped plowing the roads for the season, Mary Ann Moss, watching where she stepped on the steaming rock shelf, scooped a handful of gravel and placed it, just so, at a hole in the stone near Giant Geyser.

“Our high-tech markers,” she said apologetically in her soft Indiana twang. At her next inspection, if the gravel is washed away, she will know some form of eruption has taken place.

After all, she can’t sit there in 20-degree weather, not counting the wind chill, day and night. This time of year, three hours at a stretch is hard enough.

As to the markers, Moss actually prefers pine needles to gravel. “I don’t like the way it looks,” she says of the small, virtually unnoticeable piles.

Moss, 57, is a geyser gazer--in official Park Service terms a “thermal volunteer,” one of a couple dozen gentle souls who spend much of their summers surrounded by turquoise- and rust-colored pools and the nose-wrinkling smell of hydrogen sulfide. Their goal: to watch and measure the superheated fountains of the last great geyser field in the world.

The Park Service has the budget to keep track of Old Faithful’s activities, but not those of the more than 200 other active geysers. Geyser gazers provide the data for long-term research as well as for predictions of activity at lesser-known park attractions.

“Geysers are just so wonderful,” said Moss, the last volunteer gazer in the park this year. “We want everyone to share them,” she says with a shy smile.

One gazer, Herb Warren, 87, has come here from Denver for the last 30 years, getting up at 5 a.m. to keep track of Old Faithful in the hours before park naturalists begin their observations. He knows that some tourists will remember for the rest of their lives the spectacle of a dawn eruption, if he can help to tell them when to show up.

Before his recent death, John Railey used to take the other shift in front of Old Faithful, from the closing hour of the visitors’ center until midnight, so that tourists might learn of the opportunity to see an eruption by moonlight.

And geyser gazers alert park naturalists when an irreverent visitor tosses a log into a boiling pool.

“They’re our eyes in the geyser basins,” said Rick Hutchinson, Park Service research geologist in charge of the 10,000 assorted thermal attractions of Yellowstone.

They are also in the front line in a lonely effort to preserve geysers around the world.

“Geysers are very, very rare,” says Paul Strasser, a geyser gazer from Sacramento and secretary of the Geyser Observation and Study Assn. Geysers in Iceland, Siberia, Asia and Chile have been lost forever when geothermal plants were built nearby, upsetting the precise balance of heat, water and steam that make geysers operate. New Zealand, which once had more than 300 active geysers, is down to about a dozen.

Most environmentalists see geothermal power as a clean alternative to fossil fuels or nuclear power, Strasser noted, and so are not exactly fired up about preserving the fields.

Strasser was surprised when a field representative for the Nature Conservancy listened with interest to his plea to save a small geyser field near Reno. The conservancy hadn’t known that there were geysers on the same land they were trying to preserve as habitat for a rare form of buckwheat.

No one has suggested that Yellowstone’s geysers be disrupted by the steam wells of a geothermal plant. In fact, Yellowstone has been protected as a park and has been attracting visitors since before Lt. Col. George A. Custer stood his last stand.

Moss first came to Yellowstone in 1963 to see the bears. By 1980, she was coming back for the geysers. Now, every Aug. 17, from her home in Albany, Ind., she sets out for the park in her Ford Econoline van, equipped with a bed, table and microwave; and 1,700 miles later, she is with her geysers again.

“They are just a fabulous thing to see,” she said last Tuesday, the last day of Yellowstone’s summer season. “And I guess now you would say they have become a passion. I’m very serious about them.”

Giant Geyser had in recent years been her favorite “until I saw Daisy and Splendid in simultaneous eruptions in the winter of ‘86,” she said.

Still, Giant has been her research project since 1987. It is an extremely noisy and powerful geyser during eruption, and a legendary feature in the park. Its eruptions have been few and far between for the last three decades, but it may be making a comeback. Last year, it went off four times.

Moss has never seen Giant erupt. The last time she missed it, she was down the road, checking a geyser called Opal. “It has avoided letting me see it,” she said.

And so, as the few straggling tourists leave, making their way up the icy highway toward Bozeman, Mont., and as West Yellowstone prepares for the return of the U.S. Olympic ski team for winter training, Moss hangs on for a couple more days, just in case.