On 3 October 2023, a volume of 14.7 million m3 of a permafrost moraine slid into South Lhonak Lake at 5200 m above sea level in Sikkim, India. The tsunami-type impact wave in the moraine-dammed lake triggered deep breaching of the lake outlet, initiating an outburst flood that traveled 385 km along the Teesta River to Bangladesh, causing numerous losses of lives, destroying the Teesta III hydropower dam and heavily damaging other hydropower infrastructure, buildings, traffic connections and agricultural land.

The event with its grave and far-reaching consequences was analyzed by an international and interdisciplinary group of experts. The assessment of ongoing glacier retreat, of resulting lake expansion and of pre-failure permafrost conditions at the detachment site of the initial landslide helps with understanding the involved destabilization and tsunami-producing process. In the deeply frozen moraine, rapid and coherent creep movements with velocities up to about 10 meters per year had already taken place several years before the final event. This documents that steep moraine slopes under permafrost conditions and non-frozen moraines behave in a fundamentally different way. This is essential for assessing hazards from other expanding or newly forming lakes.

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