Danger zones are emerging across the mountain region as glaciers melt.
IT'S strangely calming to watch the Imja glacier lake grow, as chunks of ice part from black cliffs and fall into the grey-green water below.
But the lake is a high-altitude disaster in the making - one of dozens of danger zones emerging across the Himalayas because of glacier melt caused by climate change.
If the lake, situated at 5100 metres in Nepal's Everest region, breaks through its walls of glacial debris, known as moraine, it could release a deluge of water, mud and rock as far as 100 kilometres. This would swamp homes and fields with a rubble layer up to 15 metres thick, leading to loss of the land for a generation. But the question is when, rather than if.
Mountain regions from the Andes to the Himalayas are warming faster than the global average under climate change. Ice turns to water; glaciers are slowly reduced to lakes.
When Sir Edmund Hillary made his successful expedition to the top of Everest in 1953, Imja did not exist.
But it is now the fastest-growing of about 1600 glacier lakes in Nepal, stretching down from the glacier for 2.5 kilometres and spawning three ponds. At its centre, the lake is about 600 metres wide. It is believed to be up to 96.5 metres deep in some places. It is growing by 47 metres a year, nearly three times as fast as other glacier lakes in Nepal.
''The expansion of Imja Lake is not a casual one,'' said Pravin Raj Maskey, a hydrologist with Nepal's Irrigation Ministry.
The extent of recent changes to Imja has taken glacier experts by surprise, including Teiji Watanabe, a geographer at Hokkaido University in Japan, who has carried out field research at the lake since the 1990s.
Dr Watanabe returned to Imja last month, making the nine-day trek with 30 other scientists and engineers on a US-funded trip led by the Mountain Institute. He did not expect such rapid changes to the moraine that is holding back the lake.
''We need action, and hopefully within five years,'' he said. ''I feel our time is shorter than what I thought before. Ten years might be too late.''
Mobilising engineering equipment and expertise to a lake 5100 metres up and several days' hard walking from the nearest transport hub is challenging in Nepal, one of the poorest countries in the world.
People living in the village of Dingboche, below the lake, say scientists and government officials have been talking about the dangers of Imja for years.
Some years ago one visiting expert was so convincing about the risk of an imminent flood that villagers gathered their animals and valuables and moved to the next valley. They came back after a week when the disaster did not happen but say it is hard to dismiss the idea there could be a flood one day.
''When I was 21, I went to the lake and it was black and really small,'' said Angnima Sherpa, who heads a conservation group in Dingboche. ''Two years ago I went there and it was really big. I couldn't believe it could get so big. It was really scary.''
But scientists and engineers still cannot agree on whether to rate Imja as the most dangerous glacier lake in the Himalayas, or a more distant threat. Mobilising world help for big engineering projects during the global financial crisis is also hard.
The United Nations Development Program and other agencies have supported a project to drain the lakes, but those funds are running out.
John Reynolds, a British engineer and expert on glacier lakes who has worked in Nepal, says the international community has focused on Imja because of its proximity to Everest and trekking routes popular with Western tourists. He says there are other, more hazardous lakes elsewhere.
The Nepalese government ranks Imja among the six most dangerous glacier lakes in the nation largely because it is growing so quickly. More than 12 other such lakes are also seen as high risk.