India, Oct. 6 -- South Lhonak lake had pipes in it. For years, engineers had run siphons over its moraine wall to drain the water down and buy the valley a little safety. On the night of 3 October 2023, none of it mattered. A slab of frozen mountainside, about 14.7 million cubic metres of rock and ice, broke off and crashed into the lake. The water heaved back a wave some 20 metres high, tore through the loose wall of debris that had held it for decades, and sent roughly 50 million cubic metres down the Teesta.

By the time the surge reached Chungthang, 60 km below, it was deep enough to swallow the 1,200 MW Teesta-III dam, the largest in Sikkim. It did not stop there. The flood was tracked for hundreds of kilometres down the river, throu...