On 3 October 2023, 14.7 million m3 of a permafrost moraine collapsed into South Lhonak Lake at an elevation of 5,200 m a.s.l. in Sikkim, India. The resulting tsunami-like impact wave in the moraine-dammed lake caused deep breaching of the lake outlet and triggered an outburst flood that travelled 385 km along the Teesta River into Bangladesh. The flood claimed numerous lives, destroyed the Teesta III hydropower dam and caused extensive damage to other hydropower facilities, buildings, transport infrastructure, and agricultural land.

This event and its grave and far-reaching consequences were analysed by an international and interdisciplinary team of experts. Investigation of ongoing glacier retreat, the resulting expansion of the lake, and the permafrost conditions at the landslide detachment site before failure have helped to clarify the processes that led to slope destabilization and tsunami generation. The deeply frozen moraine had already been undergoing rapid and coherent creep movements, with velocities up to about 10 metres per year, several years before the collapse. This demonstrates that steep moraine slopes under permafrost conditions behave fundamentally different from non-frozen moraines. Understanding this distinction is essential for assessing hazards associated with other expanding or newly forming mountain lakes.

Detailed analyses were also carried on the flood propagation and the resulting massive erosion down valley from the lake.