As for El Capitan, every climber from Everest base camp to the Brooklyn Boulders gym recognizes it as the indispensable cliff. There are bigger cliffs on Baffin Island and smaller but steeper cliffs elsewhere, but no cliff anywhere combines such unrelenting steepness, glassy smoothness and inspiring immensity — horizontal as well as vertical — with a quality of such coherent unity, of being a single solid object so gigantic as to reliably induce a tingling awareness of creation’s incomprehensible mystery.

I can tell you from experience that first-time El Capitan climbers sometimes find its vastness and verticality so overwhelming — inducing such a heady fusion of vertigo and agoraphobia — that, even stitched to the wall by a fortune’s worth of gear, they flip out and retreat. I can tell you that when such unsteady souls do manage to reach the upper headwalls — typically after three or four days of continuous upward toil and terror and wild, screaming fun, living in filth on canned chili and candy bars — they have been known to drop daypacks full of mission-critical equipment and watch in horror as those daypacks free-fall through space for thousands of feet toward pine trees so far below they look like miniature shrubs in a model railroad diorama.

El Capitan does have vertical fractures that allow expert climbers to insert their hands and feet and sometimes entire legs and arms and then twist and flex those body parts in highly technical and painfully exhausting ways that do create a temporary grip over the abyss. Still, so much of that cliff is devoid of anything that normal human beings would recognize as handholds or footholds that the overwhelming majority of El Capitan climbers resort as I did to artificial aid. We insert hardware into cracks, clip nylon stirrups to that hardware and stand in the stirrups. Even then, we struggle to stay calm in an environment that feels like a mile-wide plain of smooth stone that we happened to be lying upon when it flipped to vertical and swung us higher into the sky than any of the world’s tallest skyscrapers yet reach. The mental strain of burying all that fear for all that time becomes its own monumental effort.

Elite climbers do “free climb” El Capitan, meaning that they make all upward progress with hands and feet on the rock, and with gear employed only as a safety net. But the nature of the rock is such that no amount of finger strength can make it feel entirely secure. Much of the terrain is so smooth that it can be ascended only by identifying half-imaginary indentations, pressing shoe rubber against those indentations — smearing, as the technique is known — and then hoping that the rubber sticks while you ease upward.

Nobody is better acquainted with this aspect of El Capitan than Caldwell, whose 2015 climb of the Dawn Wall on El Capitan, with Kevin Jorgeson, is considered the hardest long free climb ever done. “If you don’t have your body position exactly right, you can easily slip and fall,” Caldwell told me. “And if you’re at all nervous, there’s a downward spiral where you pull harder with your hands and lean in closer and your feet shoot out, so it takes incredible confidence.”