TIMING IS EVERYTHING ON GEYSER WALK.

Byline: - Eric Noland

YELLOWSTONE NATIONAL PARK, Wyo. - Anemone Geyser was dry and silent as we walked up. Our trail brochure noted that these benign traits were in fact precursors to an eruption, so we stood along the boardwalk and waited - though other tourists hurried past.

Soon enough, the show began. The pool filled, then churned with boiling water on the surface. A spout formed - 5 feet, 6 feet - belching steam. The spectacle lasted barely a minute, and then the gusher subsided as quickly as it had formed, and the water receded into the pool, disappearing with the sound of a flushing toilet.

Later, we concluded that little, dramatic Anemone was our favorite among the many geysers, hot springs and mud pots to be found inside the volcanic caldera of Yellowstone National Park, a 30-by-45-mile basin at the heart of the park.

Guests of the Old Faithful Inn can readily get to Anemone and the better-known attractions of the Upper Geyser Basin - Old Faithful, Castle, Riverside, Morning Glory Pool - on a web of boardwalk trails that strings out along the Firehole River. Trail guides are available for 50 cents at the visitors' center or at dispensers at the entrance of the route.

The science of all this hydrothermal activity is pretty simple. Volcanic eruptions occurred here in Yellowstone's ancient past, the last about 640,000 years ago. A huge chunk of land collapsed, forming the caldera, or interior of the volcano. Water on the surface seeps down into porous layers of rock, where it is flash-heated - sometimes by magma that is only three miles below your feet. The water races back to the surface as steam or hot springs.

For the visitor, it presents an otherworldly environment. Rocky cones smoke ominously (notably Castle Geyser). Some pools spurt and splash (Sawmill). Several holes have water boiling at the surface, bubbling furiously as if waiting for a handful of pasta. Peer inside and you'll see an orifice as complex as the human ear.

The water in some deep pools is bright-blue in color, because in the rainbow spectrum, blue rays scatter the most when sunlight strikes the water's surface. Also, the extreme heat of the water enhances the effect.

It was disappointing to discover that Morning Glory Pool wasn't as blue as it had been during the family vacations of my youth, and it was disheartening to hear why. Over the years, visitors just haven't been able to resist tossing objects into it - coins, rocks, logs, litter. The foreign objects caused the temperature to drop, and yellow, orange and brown algae formed in the cooler water. Despite warning signs, rangers still have to drain the pool and extract the trash on a regular basis.

Travelers who throng to the Upper Geyser Basin often miss the Biscuit Basin, a short drive away. Maybe that's why its Sapphire Pool exhibits the blue brilliance once associated with Morning Glory.

At the visitors' center, rangers post predicted eruption times for some geysers, but patience is a must. Some of the forecasts will be worded as ``within 45 minutes of ...'' or ``within an hour of ...'' At Oblong Geyser, we encountered a fellow who had decided he was just going to sit down and wait it out.

Sometimes you can just be lucky. Castle Geyser suddenly erupted when we were on the opposite side of the basin, but the spout was so robust we had a great view of it nonetheless. Elsewhere in the park, at Lower Geyser Basin, where we stopped to see the Fountain Paint Pots, we were treated to an abrupt and sustained eruption of Clepsydra Geyser - so vigorous that the steam cloud engulfed part of the boardwalk, scattering a handful of visitors.

Yellowstone's main event, of course, is Old Faithful, which spouts skyward at regular intervals (over the years they've varied from 45 minutes to 120 minutes; currently, the eruptions are spaced at about 90 minutes). The plume regularly tops 100 feet, and sometimes will soar to nearly twice that height.

If you watch this one from the boardwalk, you'll have plenty of company, but for a much different perspective, cross the Firehole River on a nearby bridge and detour onto the Observation Point Trail. The one-mile round trip climbs through an evergreen forest, where marmots play among the downed logs (we encountered several).

At the overlook, the view takes in the geyser basin, the inn - and, if your timing is right, an eye-level view of the top of Old Faithful's gusher.

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(1 -- 2) A short stroll from the Old Faithful Inn leads visitors to a series of boardwalks that stretch along the Upper Geyser Basin, where sights like Castle Geyser, left and above, amaze.

Eric Noland/Travel Editor

National Park Service