Gazing into the majestic Grand Canyon, awe-struck visitors inevitably ask: "How old is it?"

Far older than generally thought, says new evidence that scientists culled from caves lining the canyon's red limestone cliffs.

The Grand Canyon often is referred to as about 6 million years old — but its western half actually began to open at least 17 million years ago, a University of New Mexico team reports Friday in the journal Science.

Wait: The western side of the canyon is the downstream end of the Colorado River, so how could it be older than the arguably more spectacular eastern side?

Remember, geologists caution, that the Grand Canyon was carved from drainage systems that didn't turn into the single river we now know as the Colorado until roughly 6 million years ago. The new research suggests two canyons formed that eventually joined. And it makes sense that the older side would even look different, less jagged, thanks to more years of gravity and wind erosion to soften its edges.

"This is really exciting for those of us who work in the stories and theories of how the Grand Canyon has evolved," Arizona geologist Wayne Ranney, author of "Carving the Grand Canyon," said of the new work. "This paper helps us to more clearly understand that different parts of the canyon formed at different times. That's how big the Grand Canyon is."

How and when the Grand Canyon formed has been a question of both geologists and average visitors since John Wesley Powell's famous first expedition in 1869.

Dating the canyon's carving has been difficult because it has largely depended on evidence from exposed rock and mineral deposits that themselves erode over time.

The University of New Mexico team tried a new technique: Testing formations inside the numerous caves that line the Grand Canyon — protected formations less susceptible to erosion — that form at the water table. So cave specialist Carol Hill said they should provide a record of how the water table dropped over time as the canyon was cut deeper and deeper.

First Hill and colleagues made the grueling climbs to cull the formations from caves in 10 different spots along the length of the Grand Canyon. Then came work in specialized labs to pin down the age of each formation, using a method called uranium-lead isotope testing.

The findings: The western side of what is now the Grand Canyon started forming about 17 million years ago, and that initial erosion was fairly slow and steady — a couple of inches every thousand years.

The canyon formed not just downward and westward but it opened steadily to the east, too, through what geologists call "headward erosion," the team reports — until the western side cut through enough rock to meet water on the eastern side, around 5 to 6 million years ago.

Then the action really started, with the eastern side of the canyon being cut at a rate of about 8 inches to almost a foot every thousand years, they report.

Why the speedup? The new research can't say exactly, but Ranney notes that land mass was shifting around a lot during this period, too, heaving some sections of rock and lowering others. The Hurricane and Toroweap faults in the western Grand Canyon dropped enough to essentially form a waterfall, speeding water flow enough that the eastern side was being ripped as the river plunged to the west, he explained.

While geologists point to some questions in the new research, overall it does fit with various theories about how the Grand Canyon formed, said Rebecca Fowler of the University of Colorado, Boulder, who also studies the Grand Canyon.

"All of it is pointing toward a pretty complex history of Grand Canyon development, which is one of the reasons this area has been so controversial," she said. "It's a pretty complicated system and it's very likely that the entire Grand Canyon did not incise (cut) all at one time."