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(NEXSTAR) – Fifteen people died from avalanches across the country last week, making it the second-deadliest week for avalanches in the U.S. ever, according to the Colorado Avalanche Info Center, which tracks avalanche fatalities.

Alaska, California, Colorado, Montana, New Hampshire, Utah and Washington all experienced fatal avalanches over the seven-day period.

The deadliest occurred in Mill Creek Canyon, Utah, after eight backcountry skiers were caught in an avalanche. Only four survived.

The dead were all under the age of 30: 29-year-old Sarah Moughamian of Sandy, Utah; 26-year-old Louis Holian; 26-year-old Stephanie Hopkins; and 23-year-old Thomas Louis Steinbrecher, all of Salt Lake City.

The 7-day stretch is the deadliest behind only what’s now known as the Wellington avalanche, which killed 96 people in Wellington, Wash. in 1910.

The avalanche was caused by a lightning strike, which broke a half-mile-long block of snow from a mountain overlooking the town. It ran into a railroad depot, throwing the trains 150 feet downhill and killing 96 people. Twenty-three people were pulled from the wreckage and survived.

Twenty-two people have died in avalanches since the start of 2021. Last year, 23 people were killed by avalanches in total.

“We have seen an unprecedented winter,” Craig Gordon, a Utah Avalanche forecaster, told NBC Nightly News on Monday. “It’s been unusually dry but that sets the stage for a very weak, shaky foundation.”

According to the Utah Avalanche Center, wind is the most common cause of avalanches, as it erodes snow from the upwind side and deposits it on the downwind side.

Rapid warming can also lead to avalanches, though it’s “much rarer.”

In 90 percent of incidents, a victim’s party triggers the avalanche, as victims tend to be backcountry recreationalists, including skiers, snowboarders and snowmobilers.

Avalanches are hard to avoid when you’re caught in one’s path. Dry slab avalanches typically travel 60 to 80 miles per hour, and can reach those speeds in up to five seconds. Wet avalanches typically travel around 20 miles per hour.