GRAND CANYON NATIONAL PARK, Arizona (AP) – About 4.5 million people visit this natural wonder every year, with relatively few venturing very far below the rim. Down there, however, is where the trails along the cliffs offer constantly changing 3-D views that quite simply are otherworldly.

Even a short trek makes it clear why.

Temperatures in the inner canyon, a mile below the rim, soar well past 100 degrees much of the year, even though rim temperatures are cooler because of 6,000-to-8,000-ft altitudes. And unlike the typical downhill exit to most mountain ascents, the end of any climb in the Grand Canyon is always uphill – or in a helicopter if you are injured, sick or worse. More than 250 people have to be rescued every year.

"As beautiful as this place is, I think some people, perhaps, forget that it is a wild and rugged environment and, therefore, there are inherent dangers," said Shannan Marcak, park spokeswoman. For the well-prepared visitor who is relatively physically fit, hiking is safe and offers an experience that is difficult to duplicate.

The South Rim is open year-round, although the trails can be covered with snow and ice on the upper elevations during winter yet still warm on the canyon floor. The North Rim is accessible in winter by skis. Most people hike in the months before and after the midyear summer months because the desert environment is much cooler than summer's.

Even in early autumn, the high temperature hit 110 on the bottom when I set out with my wife, brother and his wife for our first trek of this natural wonder.

We went down the expansive 7.2-mile South Kaibab Trial from the South Rim, stayed two nights in one of the four-person cabins at the historic Phantom Ranch and then hiked out the 9.6-mile Bright Angel Trail, both of which connect to the North Kaibab Trail that leads to the North Rim. Bright Angel is also a good trail for short day hikes.

The trails are narrow and relatively steep but there are no vertical climbs. The hardest part is near the top.

The view is breathtaking from the rim, and the layers of earth are easy to see, but hiking through those strata and the various formations gives the visitor a close-up view. Every switchback offers a different, overwhelming scene. The changes also are visible in the different colored dust that collects on the trekker's shoes: tan, then red, then another shade of brown. "It keeps unfolding like a puzzle," my brother observed.

Geology was never so interesting.

The erosion that created the canyon deposits silt into the Colorado River, giving it a red hue much of the year.

"This river's nature is to be red and full of sediment," Marcak said.

Besides the coolness of the morning, hiking early also lets you see the shadows advance along the landscape as the sun treks across the sky.

At night, there is no light pollution on the bottom of the Grand Canyon, so the stars shine brightly and the constellations are clearly visible. We hiked out before dawn and found that headlamps are helpful but even a half-moon offers more than enough natural light to see the trail and clear views of the river that rushes far below.

It is one of those moments you take in and recall again and again after the trip: you are under the heavens on the bottom of the Grand Canyon, and the only natural sound is the rush of the Colorado River.

"The vastness of this is literally overwhelming to many people. I think some people are really struck with their insignificance," Marcak said.

One of the hikers we met on the trail was Bob Stevenson, 67, of Boss, Missouri, who rode down the Grand Canyon on a mule with his wife in 1995 and had returned with his adult son.

The mule ride was not as taxing physically, but hiking gave the father-son team a chance to linger on the trail, take it all in and spend some quality time, he said, comparing the only two ways to get to the bottom on land. "I think the hiking just gave you an opportunity to get a better feel for the canyon itself because you could stop and look. And we had the squirrels getting in our backpack and the ravens all around us. It was pretty neat," said Stevenson, a retired accountant.