A deadly cascade: A catastrophic landslide in Uttarakhand state in India on February 2021 damaged two hydropower plants, and more than 200 people were killed or are missing. Shugar et al. describe the cascade of events that led to this disaster. A massive rock and ice avalanche roared down a Himalayan valley, turning into a deadly debris flow upstream from the first of the two hydropower plants. The sequence of events highlights the increasing risk in the Himalayas caused by increased warming and development. Science , abh4455, this issue p. 300

Hard rock cliffs represent approximately 75% of the world's coastline. The rate and nature of the mechanisms that govern the retreat of these cliffs remain poorly constrained, primarily because conventional approaches employed to monitor these processes are generally inadequate for describing cliff erosion processes directly. These techniques are usually centred upon the interpretation of data collected periodically from aerial sensors, including stereographic aerial photographs and more recently air-borne LIDAR. These methods are generally not capable of assessing the pattern of erosion on the cliff face due to the oblique viewing angles, and hence tend to concentrate upon the resultant recession of the cliff top rather than change on the cliff face. Thus, processes of undercutting and small scale iterative failures of localized sections of the cliff face are generally not recorded. It is only when a failure affects the cliff top that any retreat is recorded. It is therefore unsurprising that cliff erosion is commonly deemed to be episodic. This paper presents a new approach to detailed cliff process monitoring using terrestrial laser scanning (TLS), which directly monitors changes on coastal cliff faces. The method allows the quantification of failures ranging in scale from the detachment of blocks of a few centimetres in dimension through to large rock, debris or soil, falls, slides and flows over 1000 m ³ . The collection of data is on-site and rapid and hence cost effective, providing a detailed description of the nature of coastal cliff erosion. This paper describes the methodological approach and demonstrates the range of results which can be generated, here shown for 16 months of monitoring data collected for a near-vertical cliff section on the coast of North Yorkshire, UK. The results demonstrate that terrestrial laser scanning can be used to quantify cliff failures to a previously unobtainable precision. The results reveal a strong spatial and temporal pattern of cliff collapse which contradicts commonly held perceptions of the nature of coastal cliff development.