Himalaya melting: glacial lakes pose a new peril as ice turns to water.(GLOBAL WARMING ALERT)

IN THE HEART OF NEPAL'S SAGARMATHA National Park, I hike above the hamlet of Chukung, 11 miles south of Mount Everest, with Tenzing Sherpa, a former monk in the Tengboche monastery. We wander through lush yak meadows beneath the monolithic Lhotse, the world's fourth-highest mountain peak. Ravens clack and prattle as they wheel above us, and I scan the surrounding peaks, all more than 22,000 feet. Our faces are seared by the high-altitude sun, our backs chilled by the 10-degree air.

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We scramble up the flank of a 200-foot-high hill of freshly churned rubble that stretches more than a mile. I reach the ridgeline, peer cautiously over an abrupt edge, then quickly draw back from a precipitous drop to a lake 150 feet below. I crawl again to the edge and watch a skein of late-morning mist rise from the water's surface, like steam from a cup of coffee.

Fifty years ago, Imja Tsho (tsho is Tibetan for lake)--now about a mile in length and up to 295 feet deep--did not exist. Instead of looking down at a body of water, we would have looked out at the Imja Glacier, an upended maze of house-size blocks of ice and near-bottomless crevasses. Now, between breaths, Tenzing and I listen as rocks loosened from the gravelly glacial moraine by the morning thaw clatter and plunge to the water.

The rapid melting and recession of the Imja Glacier, and the simultaneous growth of Imja Tsho, have alarmed national park staff and the Sherpas who grow potatoes and run …