The sudden, explosive collapse of the Chungthang dam, a unit of the Teesta-III hydroelectric project in the tiny Himalayan state of Sikkim on October 4, has come and gone from the world’s news pages. But the failure of the dam and its large cement hydroelectric station, which was washed away in less than 10 minutes, killing at least 22 people and leaving more than 100 missing from a tidal wave that crashed down from the high Himalayas after a cloudburst near the Tibetan border, is an ominous warning of things to come, environmentalists say.