Volcanoes are among the most rapidly growing geological structures on Earth. Consequently, their edifices suffer structural instability that may result in lateral flank collapses, such as the 1980 Mt St Helens event or the 2018 collapse of Anak Krakatau (Indonesia). The seafloor displays the geological remnants of collapses of nearly all ocean island volcanoes, including Hawaii and the Canary Islands. Such collapses and their associated tsunamis are among the largest and most disastrous natural processes on Earth, because of the enormous energy involved. Numerous coastal and ocean island volcanoes worldwide show signs of flank instability, documented by ground deformation measurements. However, it is difficult to evaluate their hazard potential mainly due to a lack of understanding of the causes of collapse. For coastal and ocean island volcanoes, most research and the vast majority of monitoring activities are biased towards the often comparatively small part of the volcano above sea level, while the largest part of the volcanic edifice is typically submerged in water. Using the example of Mount Etna (Italy) as well as several other case studies, I demonstrate that shoreline crossing analyses of volcano-tectonic structures and edifice deformation are necessary for understanding the mechanisms that control the volcano’s structural stability. I further argue that the earliest and most important precursory signals for imminent edifice collapse may occur below sea level. Data acquisition and monitoring in the deep sea is technologically and logistically challenging, but possible. It significantly extends onshore data sets with the potential to revolutionise our current understanding and hazard monitoring.
