ANCHORAGE, Alaska (AP) — Three skiers are believed dead after an avalanche swept them away and buried them under a pile of snow nearly 10 stories deep in the Alaska backcountry.

Each winter about 25 people on average die in avalanches in the U.S., with most occurring in the wilderness, according to the National Avalanche Center. They are rare at ski resorts, which manage their slopes and trigger smaller slides on purpose when no visitors are around.

Here’s a look at avalanches, how they occur and some tips on how to stay safe:

What were some recent avalanches?

The heli-skiers are believed to be dead after being swept away and buried Tuesday afternoon near the skiing community of Girdwood, about 40 miles (64 kilometers) south of Anchorage. Heli-skiing is a type of backcountry skiing that involves using a helicopter to access remote areas and slopes of fresh snow.

If the three men’s deaths are confirmed, it would be the deadliest in the country since three climbers were killed in Washington’s Cascade Mountains in 2023.

Experts said it is highly unlikely the helicopter triggered the avalanche because the vibration or noise from it doesn’t release enough force.

Prior to this week, 15 people had been killed across the U.S. by avalanches this winter: 10 backcountry skiers and snowboarders, four people on snow machines and a ski patroller, according to the Colorado Avalanche Information Center.

The most recent was Feb. 28 in the Harts Pass area of Washington, where three snowmobilers were caught in a slide. One was killed, and one was injured.

One person was killed in central Colorado on Feb. 22 by a skier-triggered avalanche in a steep area above Berthoud Pass. It was the third avalanche death in the state this winter and the second in less than a week, according to the Colorado Avalanche Information Center, after a Crested Butte snowboarder died Feb. 20 west of Silverton.

Elsewhere three people died Feb. 17 in avalanches: one person near Lake Tahoe, which straddles the California-Nevada border, and two backcountry skiers in Oregon’s Cascade Mountains. On Feb. 8 a well-known outdoor guide was killed in an avalanche in Utah.

How do avalanches occur?

It generally takes just two ingredients to create dangerous conditions: A slope of 30 degrees or more and layers upon layers of snow.

Extra pressure on top of that snowpack from weight, wind, rain, heavy snow or motion can cause some of the layers to shear off and slide downhill.

Sometimes a slide happens in the form of loose snow, called a sluff. Sluffs account for only a small percentage of deaths and property damage from avalanches, according to the Sierra Avalanche Center.

Other avalanches are made up of slabs, which happen when a large layer of snow breaks away. Those account for most fatalities.

Another kind of avalanche occurs when wind creates a cornice of snow that hangs over a ridge or the edge of a steep slope. The overhang can collapse suddenly, catching anyone underneath or on top of it by surprise, according to the Colorado Avalanche Information Center.

What triggers an avalanche?

Movement, rapidly changing weather, wind — any one of these things can set off an avalanche. But experts say 90% of slides that cause an injury or death are triggered by the victim or a companion.

That means people who ski, snowmobile, snowshoe or enjoy other winter backcountry activities should check the avalanche forecast before they head out and make sure they have the right safety gear.

Most ski resorts have avalanche protocols or mitigation systems. Often that means checking snowpack stability. Remote detonations are also used to trigger slides intentionally and remove risky buildup before skiers are allowed on the slopes, said Chris Lundy, an avalanche specialist with the National Avalanche Center.

Lundy said is highly unlikely the helicopter that dropped off the skiers in Alaska’s backcountry triggered the avalanche.

“There is a popular misconception that sounds or vibrations trigger avalanches, and that is not correct at all. There needs to be a physical force on the snowpack for an avalanche to occur,” he said.

How can one avoid an avalanche?

Don’t think for a second that you can outrun one. Dry slab avalanches typically reach speeds up to 80 mph (129 kph) within seconds, according to the Sierra Avalanche Center. Wet avalanches usually travel around 20 mph (32 kph).

By comparison eight-time Olympic gold medalist Usain Bolt set a record of just under 28 mph (45 kph) in the 100-meter dash in 2009 — and that was on a flat track, not a snow-covered slope. Most people caught in avalanches are on the slope when they occur.

The better plan is to make sure you’re not in a place where one is at risk of occurring. Outdoor enthusiasts can check forecasts at regional avalanche centers or www.avalanche.org.

If you do venture out, it’s a good idea to use the buddy system and be versed in wilderness first aid. Experts say three pieces of gear are essential: an avalanche beacon or transceiver, which sends location signals to others; a shovel to test snowpack or dig out companions; and a thin, folding pole to poke into the snow in search of anyone buried. An avalanche airbag backpack that inflates after the skier pulls the trigger giving the person a chance to stay above or close to the surface during an avalanche.

What if you get caught in an avalanche?

As fast as you can, try to cover or cup your mouth and nose with your hands. If your arms are extended when you’re buried, you won’t be able to move them.

The cupped hands provide a tiny air pocket. And your breath is warm when you blow out, so it can help melt the snow. If you’re lucky you might be able to wiggle a finger or two to slowly dig through otherwise cementlike snow for space, with more air, and eventually escape.