I’m reading Book 8 of Herodotus this morning, and he writes a passage that leads me to wonder if he’s referring to a tsunami. Here it is (from Macaulay’s translation):
“When three months had gone by while Artabazos was besieging the town, there came to be a great ebb of the sea backwards, which lasted for a long time; and the Barbarians, seeing that shallow water had been produced, endeavoured to get by into the peninsula of Pallene, but when they had passed through two fifth-parts of the distance, and yet three-fifths remained, which they must pass through before they were within Pallene, then there came upon them a great flood-tide of the sea, higher than ever before, as the natives of the place say, though high tides come often. So those of them who could not swim perished, and those who could were slain by the men of Potidaia who put out to them in boats. The cause of the high tide and flood and of that which befell the Persians was this, as the Potidaians say, namely that these same Persians who perished by means of the sea had committed impiety towards the temple of Poseidon and his image in the suburb of their town; and in saying that this was the cause, in my opinion they say well.”