This story appears in the December 2018 issue of National Geographic magazine.

Summer scenery is luminous in Yellowstone National Park, but under the surface lies an alternate reality. Photographer Brian Skerry entered the otherworldly ecosystem of Yellowstone Lake to explore unique spires formed by dormant hydrothermal vents thousands of years ago. Guiding him was Brett Seymour, pictured here, a diver and photographer for the National Park Service.

View Images Yellowstone National Park straddles Wyoming, Idaho, and Montana. NGM MAPS

T MINUS ONE WEEK

A change of altitude: As a kid, Skerry opted for rock kits over chemistry sets and hoped to become a geologist. Diving amid 11,000-year-old underwater hydrothermal formations was a dream assignment. But he had just returned from chasing dolphins in South Korea and was concerned about swapping sea level for some 7,700 feet above it: Yellowstone is the largest lake in North America at such a high altitude. How quickly would his body adjust? And in the depths, would he have the visibility for a good shot?

T MINUS THREE DAYS

Essential packing list: To insulate himself from near-freezing water, Skerry wore a dry suit, which, unlike a wet suit, has room for thermal layers underneath. By the time Skerry dived off the boat, he was wearing a hundred pounds of gear.

A dry suit

A diving harness with 30 pounds of lead weights

Wool socks and thermal pants and shirt

Nine portable underwater lights

Eight cases of camera equipment

T MINUS ZERO MINUTES

Ready for launch: Every day for a week, Skerry and Seymour would take a boat onto the lake, gear up, and roll overboard. They’d descend into what felt like a parallel universe of monochromatic shade. The water was dark and murky, so Skerry needed a lot of lighting to show a 26-foot spire. He and Seymour brought down nine lights to set up around the spires as studio lighting. It took a week of adjustment to get the right lighting for this shot.

BY THE NUMBERS

7,732

Lake’s altitude in feet

32°F

Winter water temperature