VERLOT — The Big Four Ice Caves that collapsed Monday, killing one person and injuring five, are natural phenomena that Puget Sound-area residents have been visiting for decades.

But despite their persistence, the caves are always changing, sometimes to deadly effect. Big Four refers to the mountain, where a snowfield sometimes appears in the shape of a numeral four. The caves below the north face of the mountain are not made of pure ice, the way a cave in a glacier might be.

Rather, the caves are the result of avalanches filling the gully with snow. The snow, sheltered from direct sunlight by Big Four Mountain, persists year-round.

The caves are then carved out by meltwater streams and are expanded by an inrush of warmer air under the snow.

And that’s where the danger lies.

“As ice caves grow, the roof of the ice cave thins and weakens,” said Bernard Hallet, a glaciologist with the University of Washington’s Glaciology Group.

That increases the danger of a snow bridge or cave roof collapse, Hallet said, but it’s compounded by the fact that the water flowing into the snow makes it heavier and even more prone to collapse.

The caves have been a popular tourist destination since at least 1921, when brothers Wyatt and Bethel Rucker of Everett opened a resort called the Big Four Inn on the spot where the trailhead parking lot is today, according to an essay on Historylink.org. The inn burned down in 1949.

Adrienne Hall, manager for the Verlot Ranger Station, said the early heatwave this year has made the area more dangerous than usual.

“Like with fire season, we’re a month early for this time of year,” Hall said.

The ice has grown thinner.

“There’s also cracking going on inside, further in,” she said.

Winter’s low snowfall and the recent record heat aren’t helping matters.

Hallet’s research has taken him to Alaska and Antarctica, where he has had to access areas underneath glaciers through ice caves.

It’s always risky, especially near the entrances where the ice is thinnest, he said. In mountainous regions, the risk isn’t just from falling ice but also from rocks or boulders that can fall through the snow.

“I’ve always been really careful,” he said.

Ice caves are common in the mountains, although they can be created by different mechanisms. One of the more well-known systems, the Paradise Ice Caves on Mount Rainier, were carved out from under the Nisqually Glacier by snowmelt.

The Paradise Ice Caves melted away in the early 1990s, probably for good, when the glacier retreated.

There are records of ice cave fatalities at Paradise going back at least to 1915, when C.W. Ferguson poked the roof of a cave, triggering a collapse that killed him.

Scott Beason, the park geologist for Mount Rainier National Park, said ice caves often form at the edges of the glaciers, and there are caves at the summit created by internal heating in the volcano’s crater.

That doesn’t make them any less dangerous.

“Any ice formation that is around right now is particularly unstable because of the low snowpack we have,” Beason said. “The heat is particularly bad for it.

“I don’t feel comfortable going into an ice cave even when it’s stable,” he added.

Kim Brown, of Seattle, is an avid hiker and backpacker who volunteers as a wilderness ranger with the Mount Baker-Snoqualmie National Forest. The 1.1-mile trek to the ice caves is one of her favorites.

“It’s a nice, easy hike,” Brown said. “It’s beautiful. Flowers in summer, mushrooms in fall.”

The trail passes through swaths of trees broken off partway up the trunk from avalanches. That happens when falling snow protects the lower portion of the tree, but an avalanche and its shock waves snap off the upper part.

“You walk through that before you get to the ice caves so you can see the dynamic changes of the landscape,” she said.

Approaching the caves, the end of the trail is obvious, she said. The multiple warning signs make it clear that it’s unsafe to venture further.

“When it comes to seeing a sign or seeing a rule, it’s good stewardship to practice what the agency asks us to,” she said.

Chris Winters: 425-374-4165; cwinters@heraldnet.com. Twitter: @Chris_At_Herald.