Scientists in Nepal call them the "tsunamis from the sky". Nearly 50 glacial lakes, high in the Himalayas, could soon burst their banks, according to leading mountain experts, sending millions of gallons of debris-filled floodwaters hurtling down valleys and putting tens of thousands of lives at risk. "Glacial lake outburst floods are like tsunamis in the sense that your warning is just a matter of minutes," says J. Gabriel Campbell, director-general of the International Centre for Integrated Mountain Development in Kathmandu. Posters along trekking valleys warn tourists of the danger from the "killers from the hills".