Every avalanche starts with a snowflake.

The shape of the snowflake, what happens once it hits the ground, the weather that precedes it — all these things can contribute to whether the flake becomes part of a destructive wave of snow rushing down a mountain.

Most avalanches occur after a big snowstorm, a period of strong winds or a rapid increase in temperature.

Take a slice of the deep snow, beneath where that snowflake landed. Its walls will look like layers of a cake, and each layer tells you something about the season’s previous storms, droughts, warm-ups and wind events.

As snow accumulates on the ground throughout winter, water vapor moves across the snowpack.

“Once snow falls to the ground, it’s always in a state of change,” said Brian Roman, a ski patroller at Winter Park. “It’s always moving in different directions when it hits the ground.”

Snow is Earth’s winter blanket, an insulated layer that is, counterintuitively, warmer near the ground. Near the top layer, the air temperature above the snow changes, forcing water vapor to rise and fall through the depths of the snow.

This vapor transport changes the structure of a snowflake, in a process known as snow metamorphism. Over time, this process, combined with additional weight from new or wind-redistributed snow, is the prime culprit in slab avalanches, the most deadly and most common kind.

In a safe, stable snowpack, our snowflake will tumble into microscopic round balls that are like tinier foam beads in a bean bag chair. That’s where the snow can condense and bond.

However, if there has been considerable water vapor transport, a layer of square snowflakes called faceted crystals can form a weak layer. These snow crystals have strong angular edges that resemble squares or cups, and once stacked on each other, they create air gaps, like in the middle of a Jenga game.

As fresh snow falls on top of this layer of snow crystals, it is supported as it builds. Picture it like a large book lying flat atop the Jenga blocks.

It might seem stable at first; now, imagine you tilt the table to 30 to 50 degrees, the standard slope on which an avalanche might occur.

Below 30 isn’t steep enough to make a difference, and above 50, the snow has difficulty sticking. “Thirty-eight degrees is like the magic number in Colorado for slope angle,” Roman said.

Even if it doesn’t trigger naturally, a human walking, skiing, snowboarding, snowshoeing or snowmobiling nearby can make the weak layer collapse.

This is a sudden drop in the upper snowpack, caused by the fracture of a lower layer. It makes an audible sound, which, according to experts, sounds like “whumpfing,” as air is expressed from the collapsing weak layer.

Once the slab is released, snow and ice, along with the timber and rocks they dislodge, can tumble violently down a mountainside. Sometimes, barreling snow can create a powder cloud that can travel more then 180 mph and cause significant damage on its own. The debris will break your bones, and if you survive that the snow will often suffocate you in about 15 minutes.

This season, more than 20 people died in avalanches in the United States. I went into the mountains with avalanche specialists working to keep that from happening.

Nate Birdseye, an avalanche tech at the Steamboat ski resort, is standing at the top of a mountain, about to light an explosive. He tells me to plug my ears and open my mouth.

“You may get concussed,” he says, as he lights a fuse and hurls it over the cliff edge. Inside ski resorts, mitigation efforts like this make the terrain more hospitable and ultimately less dangerous for skiers, at least the ones who stay in bounds.

Long before the resort was flooded with guests that morning, with the day’s first light shining straight toward us, we had crowded into a warm ski patrol hut on the mountain, joined by Mudd, one of the team’s rescue dogs. It’s a rustic haven where the patrollers can get out of the cold.

Old ski maps are tacked to the walls like in a teenager’s dorm room, and vintage board games are stacked on a shelf.

There’s a common area that has a quaint black wood stove and a stack of wood, but there is no fire. Sharon Spiegel, a member of the ski patrol, says it’s best not to light the stove until we’re done preparing the explosives.

I follow Birdseye into the concrete basement, and he pulls an explosive about the size of a soup can from his backpack.

This isn’t the largest explosive Birdseye will toss, but it is enough to move a small, unstable snow slab before it can become a larger, more dangerous one. As he pulls out the fuses from another part of his backpack, he explains that the primary explosive is very stable, but the fuses have explosive caps on one end. If those caps were to ignite by themselves accidentally, it wouldn’t do much, but an accidental charge next to the explosive would make for an unwelcome surprise in his bag.

He steadily inserts the caps into the explosive. When the work is done we hurry up the stairs to a higher peak to release it before the resort opens the mountain for the day’s guests.

This unique mountain terrain and large population of people who spend time outdoors have always made Colorado a hot spot for avalanches — and the deaths they can cause. February is the peak of the winter season and also historically one of the deadliest for avalanches in the United States. This year wasn’t much different: After a series of large snowfalls out West, more people died in avalanches across the country during February than in the preceding three months.

Avalanches are common in the backcountry, but some this season have tumbled through ski resorts. In December, Steamboat had its largest in-bounds slab avalanche in almost two decades. In early February, an avalanche swept into a chair lift at Montana’s Big Sky Resort. A few days later, after 6 feet of snow fell in 36 hours at Mammoth Mountain in California’s Sierra Nevada, two ski patrollers who were working on avalanche mitigation were swept away; one died.

As winter sports became more popular in the second half of the 20th century, the number of people killed in avalanches increased steadily for decades. But over the past 10 to 15 years, even as more and more people ventured out, the number of deaths started to level out, said Ethan Greene, director of the Colorado Avalanche Information Center. (One notable exception was the winter of 2020-21, when the pandemic prompted an unprecedented surge in backcountry activities like hiking and skiing in areas not necessarily controlled by a ski area.)