At 1440 utc (1540 local time) on Saturday 18 June 2022, when low tide had passed and the tide was due to rise, the water drained from the harbours of Union Hall and nearby Courtmacsherry on the south coast of Ireland (see Figure 1a). Eyewitnesses captured the event on video,1 with observers remarking that it reminded them of reports of the tide receding ahead of the tsunami in Japan in 2011. The event was captured by the Irish Marine Institute’s tide gauge in Union Hall, which recorded a drop in water level of 70cm in 5min. Compared to the normal ebb and flow of tides of ~1cm per minute, this was a dramatic event. The event impacted most of the Irish south coast, from Castletownbere to Dunmore East and onwards to Britain, with reports of unusual tidal behaviour in Pembrokeshire in South Wales (see Sibley, 2022, this issue).