Recurrent blasts sometimes tear the lid off Avachinsky (foreground), distinguishing it from the quieter Koryaksky. Both watch over the peninsula's largest city, Petropavlovsk-Kamchatsky, a fishing center whose 200,000 residents live in buildings made of a concrete rich in volcanic cinders. With some of the world's most powerful and frequent eruptions scarring this 750-mile-long (1,200-kilometer-long) swath each year, Kamchatka's 29 active volcanoes perform the greatest show on the Pacific Ring of Fire.

Carsten was ecstatic. When he and Franck decided to crawl under the glacier into a dark ice cave carved by a river of warm, acid water, I followed. Feodor just shook his head.

We crab-walked under huge blocks of ice that had fallen around the entrance, then waded through shallow water to the edge of darkness. Pale light shafted down from crevasses in the roof, barely illuminating a world of gray: gray shadows, gray ice, gray volcanic ash, gray river. The inner walls, scalloped by steam and flowing water, were hung with icicles.

The ice groaned above and around us—the internal workings of the glacier as it melted and moved. The hairs on my neck rose, and with them dreadful imaginings. Not only could the tunnel implode at any moment but also the lake, held back by only a wall of ice, could drain in a flash. It looked as if part of the cave had collapsed a few weeks earlier—what if another eruption, or even a slight earthquake, occurred while we were down there? As Carsten cheerfully put it, "The lake is above you, of course. You should feel as in a mousetrap."

As Carsten and Franck's flashlights winked out of sight ahead, I did what any prudent mouse would do. I made my way back to open ground and sat with Feodor on a dusty block of ice. Roiling sulfurous vapor filtered the sun with the hint of violence, a reminder that this peaceful afternoon was just a brief respite from the ongoing storm of rocks and fire. Feodor and I chatted about our lives and our families as the sun sank toward the crater rim. I was content just to be there, sitting on the sidelines as the gomuls did their work.