From 2019 to 2022, for the first time in Svalbard, the rapid development of a thaw slump was observed in Hollendardalen Valley (Nordenskiöld Land, West Spitsbergen), affecting an area of 6300 m2. Fast-paced thermokarst and thermo-erosion processes exposed massive ground ice, as well as thick ground ice veins within frozen silt strata. In the riverbed – in a non-carbonate, non-karstifying geological setting – thaw funnels appeared, swallowing part of the river flow, presumably via a local fault zone connecting to deep aquifers. The exposed ground ice has extremely low mineralization, dominated by Na+ and SO42− ions. The properties and morphology of the ice veins point to segregation origins. The broad middle reaches of the Hollendardalen Valley exhibit thermokarst depressions and lakes, tabular terrace remnants and traces of past thaw slumping. Such morphology represents a thermo-erosional plain, formed through the interplay of fluvial erosion and a series of fast-paced thermo-erosion and thermokarst events. The very presence of massive ground ice in places where its appearance was previously unexpected indicates the possibility of detecting further ground ice of various thicknesses in Svalbard. Thus, ongoing and future permafrost warming will likely accelerate rapid permafrost thaw in Svalbard, reshaping the surface morphology and subsurface hydrology. © 2024 Elsevier B.V. and NIPR