When Sir Edmund Hillary made his successful expedition to the top of Everest in 1953, Lake Imja did not exist. But it is now the fastest-growing of some 1600 glacial lakes in Nepal, stretching down from the glacier for 2.5km and spawning three small ponds.

At its centre, the lake is about 600m wide and, according to government studies, up to 96.5m deep in some places. It is growing by 47m a year, nearly three times as fast as any other glacial lake in Nepal. ''The expansion of Imja lake is not a casual one,'' said Pravin Raj Maskey, a hydrologist with Nepal's ministry of irrigation.

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 done field research at the lake since the 1990s.

Watanabe returned to Imja in September, making the nine-day trek with 30 scientists and engineers on a US-funded expedition led by the Mountain Institute. He said he did not expect such rapid changes to the moraine holding back the lake. ''We need action, and hopefully within five years,'' Watanabe said. ''I feel our time is shorter than what I thought before. Ten years might be too late.''

Unlike flash floods, a glacial lake outburst is a continuing catastrophe. ''It's not just the one-time devastating effect,'' said Sharad Joshi, a glaciologist at Kathmandu's Tribhuvan University, who has worked on Imja. ''Each year for the coming years it triggers landslides and reminds villagers that there could be a devastating impact that year, or every year. Some of the Tibetan lakes that have had outburst floods have flooded more than three times.''