With each step, my boots kicked up little clouds of yellow dust. The desert sun shone brightly, pushing my shadow along in front of me. The click-click-click of my friends' walking sticks marked the pace.

My eyes and ears wanted to stay on those details because when I looked up a great void opened before me, and it made me dizzy. From where the four of us walked, just below the rim of the Grand Canyon, it was nearly a mile down to the Colorado River, and 10 miles to the North Rim. In between, an abyss yawned, ringed by striated cliffs that seemed to vibrate with color.

When walking, it was a lot easier to look at my feet.

Proportion, perception and scale are all knocked out of kilter in the Grand Canyon. In 1540, Garcia Lopez de Cardenas and his party of men became the first Europeans to lay eyes on it. Standing on the rim, they estimated the river in the gorge to be about 6 feet wide, and that the rocks below were as big as men. Cardenas sent three fellow explorers down to the river, thinking they'd be back shortly. They returned at the end of the day, exhausted and bewildered, saying that the rocks were bigger than the cathedral of Seville, and that they didn't even get close to the river.

Led by Grand Canyon Field Institute instructor Lisa Kearsely, our party of four intended to do what the Spaniards couldn't -- walk down to the Colorado River in a day. Two nights later, we'd walk out.

Kearsely, a biologist and former park ranger, would guide us through the hazards of desert hiking while teaching us about an icon of the American landscape.

The Grand Canyon has changed much since the Conquistadors arrived. The famously clear air is sometimes tainted with smog from as far away as Los Angeles. The Colorado River has been dammed, drained and tamed. Nearly 4 million visitors each year add crowds, lines and parking hassles. The canyon, like most remaining wilderness areas in the Lower 48, is encroached upon from all sides. And yet, from the panoramic vistas to the jewel box-like wonders of the inner gorge, the Grand Canyon retains the power to shake one's soul.

The canyon also remains a dangerous place for fragile, water-dependent creatures such as human beings. Kearsely had spent a good part of the previous day explaining how we would hike from rim to river and back again. The Grand Canyon Field Institute, an independent nonprofit organization in the park, offers learning vacations year-round.

The one I chose last August was called "Mule-Assisted Backpack"; we'd each be sending 30 pounds of camping gear down the gorge on the early-morning mule train; we'd leave later, carrying 20 to 30 pounds, including water, on our own backs.

The Paiute called the plateau Kaibabits -- mountain lying down. For hikers, the canyon is like an inverted mountain. First the descent, then the ascent.

"That fools a lot of people," Kearsely said. "It seems so easy until you start back up. Every year people die because they underestimated it."

The combination of high altitude and desert heat can be the undoing of even a fit athlete. The climb out of the canyon is a feat similar to summit day on Mount Rainier, with an elevation gain of nearly 5,000 feet. Nine people died on the canyon's trails in 2004, and more than 250 had to be rescued, most because of heat exhaustion.

Stepping back in time

Other potentially fatal hazards covered by Kearsely included falling, lightning strikes, rattlesnakes, scorpions, drowning and hyponatrimia, which is when you drink too much water without replenishing minerals and salt.

Heeding her advice, I carried more than a gallon of water and a supply of salted nuts. My fellow travelers were two women, a psychologist celebrating her 60th birthday and a 41-year-old audiologist, both coincidentally from New York. Kearsely, 44, lives in Flagstaff. She's a lean, plainspoken mother of two. She let us set the pace, following close behind, often stopping to offer insight.

"They say with every step you take into the canyon, you're going back 20,000 years," Kearsely said, pausing in the shade by a sheer wall of yellow stone. "By the time we get to the bottom, we'll be looking at rock that's 2 billion years old."

"Take a closer look here, you can see all kinds of sea life," she said. She pointed out little fossil seashells and remnants of coral that lived in this part of the continent 250 million years ago when it was covered by an inland sea.

The next layer down, a few million years deeper in the past, was an even paler yellow, which Kearsely pointed out was an area of calcified sand dunes. "It was a free-blowing area of dunes bigger than the Sahara," she said.