He teetered backward off the mountain in silence, a 2,000-foot void sucking at his heels. There was no scream, no terrified look on his face — just quiet resignation, as if in those quick seconds he accepted he was about to die.

Dallas Kloke, my 71 year-old climbing mentor and father figure, lost his life nearly seven years ago on terrain so mellow that he and his four partners had just unroped.

“Solid as a rock,” they say.

A large block came loose in his hands and it tumbled down with him into a remote valley in Washington’s North Cascades.

Turns out rocks aren’t always so solid. Nothing is, really. Even the tallest mountain on Earth is a slave to entropy.

On May 19, climbers confirmed Mount Everest’s infamous Hillary Step — a 40-foot rocky cliff which formed the final obstacle before the summit — has fallen off. This complicated section of Everest’s regular route probably collapsed during Nepal’s massive earthquake in 2015.

For Everest hopefuls this is good news. At least this season, when a snow ridge has allowed relatively easy passage around where the Hillary Step once stood like a sentinel, guarding the top of the world.

Other stories of “permanent” rock features crashing down litter the talus of climbing lore.

In July 1983, British alpinists Joe Simpson and Ian Whittaker were bivouacked partway up the classic Bonatti Pillar on Les Drus in the French Alps. “I settled myself down on the comforting solidity of the ledge,” wrote Simpson in a 2005 article on independent.co.uk. “Seconds later there was a heart-stopping downward lurch accompanied by the thunderous sound of tons of granite plunging into the abyss.”

Their ledge for the night — on which countless climbers had slept over the decades — had fallen off! The pair dangled side-by-side, 2,000 feet up, attached only to a fragile anchor.

“Christ! Where’s the gear?” Ian asked, horrified.

“It’s gone,” said Simpson. “The hardware, boots, everything. It went with the ledge.”

They hung there helpless, and ropeless, for 12 hours until a helicopter winched them to safety.

Simpson and Whittaker’s epic foreshadowed the summer of 2005, when the entire Bonatti Pillar, home to many climbing routes up to 1,600 feet tall, disintegrated in a series of dramatic rock avalanches.

Mountains closer to home have crumbled too. The Northcutt-Carter in Rocky Mountain National Park met a fate similar to the Bonatti Pillar. Touted as one of the Fifty Classic Climbs of North America, it was Hallett Peak’s most popular climb … until the first third of it collapsed in July 1999.

The route still goes but it’s much more difficult. Climbers now avoid the rock scar via the cleverly-dubbed, “Northcutt-Harder” variation.

In Black Canyon National Park, periodic rockfall on The Serpent has bewildered climbers when their route map hasn’t matched the topography.

“When I climbed The Serpent in 2005 there was a belay in the middle of nowhere,” Josh Wharton emailed. The anchor was accessible from a ledge that had since fallen off, à la Les Drus.

Since Wharton’s ascent, a few perplexed climbing parties have backed off The Serpent when they realized two more pitches have completely disappeared.

“I was shocked,” Wharton wrote, “almost to the point of disbelief, because these features seemed huge and solid.”

Life presents many façades that appear rock solid. I’ve held onto some of them. Like my faith in God, which vanished in college. Or my first marriage, which fell apart a few years later. Or even life itself, which feels everlasting until family and friends die and we realize our time will come too.

I naïvely thought I’d climb every mountain with Dallas. Why wouldn’t we? I had never internalized that our partnership — like everything — would one day end.

Impermanence can feel crushing. Hopeless even.

Yet it can also trigger our vitality. Like jumping into an ice-cold lake, or telling someone you love them, reminding us that now is the time to live.

Contact Chris Weidner at cweidner8@gmail.com