The huge cavities beneath floating Antarctic ice shelves have only been explored recently by autonomous underwater vehicles (AUVs). Oceanic waters above the in situ freezing point melt those faces of marine-terminating glaciers and ice shelves with which they come into contact. This, in turn, impacts the dynamics of ice sheets as the reduction in buttressing allows the ice to flow faster into the ocean, increasing their contribution to eustatic sea-level rise (Shepherd et al. 2012). Pine Island Glacier (PIG) in West Antarctica (Fig. 1a, d) is an example, and has been accelerating and thinning over past decades, and still appears to be in retreat. This is driven by unpinning from a seabed ridge and exposure to high ocean temperatures of up to 1°C at the grounding line (Jenkins et al. 2010) that are subject to climatically forced variability (Dutrieux et al. 2014a).