A magnitude 7.5 earthquake struck the neck of the Minahasa Peninsula in Central Sulawesi in 2018, and the tsunami and the ground liquefaction that followed killed more than 4,300 people in and around Palu. Ten thousand or so were injured. Whole neighbourhoods on the edge of the city, Balaroa and Petobo among them, were carried along on soil that behaved like liquid.
The water reached the head of the bay within minutes of the shaking, which is almost no time at all for a warning to travel through any system built by people. Indonesia's network of deep-ocean buoys had not been reporting for years by then, for want of maintenance and funding. What did work was the tide gauge at Pantoloan, across the bay at the port, which kept recording through the whole thing, and the staff who read it afterwards. The record it left is one of the better measurements anyone has of that wave.
The graves at Poboya were opened within a couple of days, which matters in a mostly Muslim city: the custom is to bury quickly, ideally before the next sunset, and the people washing and shrouding the dead were working against both that clock and the numbers. Palu has marked the 28th each year since, with prayers at the hill and by the bay.
Liquefaction is loose wet sand losing its grip: the shaking squeezes the grains together, the water between them takes the load instead, and for a minute or two the ground carries no weight. At Petobo and Balaroa the slope was gentle, only a degree or so, but that was enough for the soil to flow and take the houses with it.