Its creation began exactly 40 years ago, on May 18, 1980, the day Mount St. Helens exploded in the Cascade Range in Washington state. The eruption shaved 1,314 feet off the mountaintop. The force sent billions of tons of wet earth rushing down the North Fork Toutle River. A plume of ash shot 15 miles high, at 300 miles per hour. All that heat and pressure left behind a hole, a massive dark crater trapped in shadows at more than 6,200 feet—a brand new factory for snow and ice.

Shielded from sun much of the year, this horseshoe-shaped depression gave rise, as winter snows accumulated beyond the capacity of summers to melt them, to a young glacier. It’s now 660 feet