Terraces and semi-circular depressions in the seafloor along islands in the southern Stockholm Archipelago, Sweden, were mapped in the 1990s with single-beam echo sounder and side-scan sonar equipment (Söderberg & Flodén 1991, 1995). After inspection by divers, these features were interpreted to be formed by processes related to groundwater that disharged at the seafloor after flowing through permeable layers in glacial clay (Söderberg & Flodén 1991, 1995). Erosional features related to submarine groundwater discharge (or sapping/piping) in the seafloor have been identified in many places and at a number of scales. For example, terraced walls and irregular courses of valleys in the continental slope of New Jersey are interpreted to be formed by groundwater sapping during periods of lower sea level (Robb 1984). The contribution of fresh groundwater to the sea is identified as an important component of the global hydrological cycle (Church 1996; Post et al. 2013) and also may contribute as much as 6–7% of the total freshwater discharge to the World Ocean (Zekster 2000 …