The surface of Rainier is dramatic enough, but the volcanic processes that created the mountain also carved whole cities of caves inside it. The fumarole caves that open onto the summit crater run for miles, some hundreds of feet deep, and are filled with potentially deadly CO2 gas. They release steam into the summit caldera, creating an atmospheric mist that creeps through photos of happy climbers posing in triumph.

Recreational climbers have ventured into the summit caves—the first recorded ascent of Rainier included a rest inside a cavern’s too-hot entrance—but are usually quickly turned by the temperatures and gases. In 2015 and 2017, members of the Oregon-based Glacier Cave Explorers led an expedition to research the dark and dangerous interior of Rainier, asking local mountaineers to help haul almost 2,000 pounds of gear up for a 10-day stay on the summit.

Using CO2 monitors and rebreathers, ice climbing gear, and a group doctor, the team collected microbiology, geology, and climatology data. They even tested a piece of equipment being developed for a NASA rover—the labyrinth of scalloped ice is so unworldly, it can stand in for another planet.