At times during a visit to Yellowstone National Park you could imagine you're in Disneyland.

It happens at the major sites, like Old Faithful, the most famous geyser in the world. Busloads of people gather on the park's grand semicircle of triple-deep wooden benches to await the somewhat predictable eruption. While waiting, people chat, lick an ice cream cone or sip a cool drink. It's a grand scene in the nation's first national park, founded in 1872, but tranquil it isn't.

For a closer look at the park's thermal features in a quieter setting, we headed to the Norris Geyser Basin. For whatever reason, most park visitors skip this site, which is the park's most vigorous thermal area. It's home to the park's tallest, erratically active geyser and possibly its hottest subsurface temperatures. Scientists measuring temperatures in the basin in 1929 had to quit when their drill rig reached 265 feet and came close to crumbling in the 401- degree heat. One park report states the highest recorded temperature in the basin reached 459 degrees at just over 1,000 feet below the surface.

We arrived at the Norris Basin early on a mid-September morning and happened into the only ranger-guided walk of the day. The area was named after Philetus W. Norris, the park's second superintendent and the first person to explain the area's thermal features.

As our little group headed out on the wooden walkway, we were reminded to not veer from the path. We were walking in the mouth of a volcano. Its landscape has the eerie, sci-fi beauty of an alien planet. All around us, steam plumes rose from multiple vents (fumaroles) in the vast open space dotted by scraggly pine trees. Some bubbling pools have a mesmerizing azure hue that deepens as you peer into their deepening layers. Others are boiling mud pots. Sounds of a blast furnace poured from scattered ground holes that propelled streams of steam. Scattered ashy-black openings lay dormant – for now.

"Nothing stays the same here," the ranger said. "We're really walking in one huge caldera" of a volcano that's still active, he says by way of introducing the basin's features. That's a bit unsettling. Even if this giant volcano hasn't had a major eruption in centuries, it could.

The Norris Basin has two loops of elevated walkways. Their smooth surfaces and gentle inclines make visiting easy for everyone, including people with walkers and baby strollers. High rails corral fleet-footed children. The park service is constantly repairing sections that rot from heat and moisture and building new stretches to accommodate the basin's ever-changing steam vents and geysers.

Both trails meander through the most active and unpredictable geothermal area of the park. Changes can happen overnight; a calm, clear blue pond can be a boiling, muddy mess the next day. Why such changes occur and then disappear remains one of the park's mysteries.

The shorter trail, the .75-mile Porcelain Basin loop, takes about 45 minutes. Our group took the longer one, the Back Basin loop, which is about 1.5 miles.

Back Basin has seven major sites with explanatory signs that help visitors who can't get to the midmorning tour. They will want to start at the compact Norris museum at the trailhead. Its explanatory exhibits make it easier to understand the region's geothermal geology.

Before we get to the largest geyser in the world, which is the second stop in the Back Basin, we come to Emerald Spring. This 27-foot-deep pool shimmers with a gorgeous imperial jade color. Its deep, pure green comes from the sulfur combined with the water's reflection of the blue light.

At Steamboat Geyser, the ranger explains how lucky we are to visit after one of its long periods of inactivity. It's blowing off plenty of steam, although its jet-propelled fountains were more like 50 feet high instead of its recorded ceiling of nearly 380 feet. There's no telling whether it will rumble to life next season.

Echinus Geyser is our fourth stop. Pronounced e-KY-nus, this geyser, as acidic as vinegar, used to be a regular gusher, sometimes spewing continuously for more than 100 minutes. But it's been mysteriously quiet and unpredictable.

That it's acidic is rare. In fact, most of the world's acidic geysers are in the Norris Basin, according to the National Park Service.

Other stops along the path include Green Dragon Spring, an intriguing name for a cave over a stream that's usually filled with steam; Porkchop Geyser, which blew its top a while back and now just perks along at a low level; and Minute Geyser, which demonstrates how man's carelessness can ruin a feature of nature. Early park visitors tossed so many rocks and coins into its vent that it now it struggles to spew forth. To fix it would damage it even further.

VISITING YELLOWSTONE

To plan a trip: If you want to overnight inside the park, you need to make reservations at least six months in advance. When you do, also make dinner reservations where possible. There are many tour groups here, so reservations are a necessity.

Timing: If you can go before Memorial Day or after Labor Day, the crowds will be lighter. However, even though the park is open year-round, Norris Geyser Basin and other secondary sites are closed from first snow to snow melt, about November through March or April.

• For a virtual tour of Norris Geyser Basin, go to this national park site: mms.nps.gov/yell/ features/norristour

• For more information on Yellowstone: http://www.nps.gov/yell/naturescience/geothermal.htm

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