A member of the expedition walks on the caldera’s cooled lava floor, turned red by the reflected glow of Nyiragongo volcano’s lava lake.

Photographer Carsten Peter tests a thermal suit in Nyiragongo volcano. “It can protect you from the radiant heat, but if you get hit with a lava splatter, the force will likely kill you,’ he says.

Constant bubbling sends waves of lava lapping over the rim of Nyiragongo’s lava lake.

With temperatures around 1800°F, the lava lake is wildly erratic. As molten rock meets the air, it cools and forms plates on the lake's surface.

A canyoneer endures the deluge of a waterfall in Empress Canyon. Canyoneers say even a relatively easy rappel like this one can feel like drowning in midair.

A canyoneer descends by rope through one of Kanangra Main Canyon’s three 150-foot waterfalls.

Midday shafts of light intensify the cathedral-like atmosphere of Rocky Creek Canyon.

A giant cave column swagged in flowstone towers over explorers swimming through the depths of Hang Ken, one of 20 new caves discovered last year in Vietnam.

A half-mile block of 40-story buildings could fit inside this lit stretch of Hang Son Doong, which may be the world’s biggest subterranean passage.

Hang Son Doong’s airy chambers sprout life where light enters from above—a different world from the bare, cramped, pitch-black spaces familiar to most cavers.

Navigating an algae-skinned maze, expedition organizers Deb and Howard Limbert lead the way across a sculpted cavescape in Hang Son Doong.