Tucked away in the 8th volume of Herodotus' Histories is a reference to a town that was saved from attack by the Persians when the sea retreated — then returned higher than ever, drowning those who tried to cross the shallows. This account of 479 BCE is regarded as the first historical reference to a tsunami — and now we have the archaeological evidence to back it up.
Urania, Book 8, 129, describes the event during the Greek Persian war, saying:
129. He then in such a manner as this had been discovered; and 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. The survivors of his army Artabazos led away to Thessaly to join Mardonios. Thus it fared with these who escorted the king on his way.
That's a description that sounds a lot like what we would call a tsunami. Now, at the Annual Meeting of the Seismological Society of America, researchers presented archaeological evidence to back this up — not only confirming the veracity of the history, but also raising questions about the safety of the region today.
The researchers took a series of cores around the Thermaikos Gulf, near Thessalonica. At a depth equivalent to the 5th century BCE, they found evidence of a "high-energy" marine event — which means big waves. They discovered large amounts of ceramics, and marine shells dating back to 2500 years ago, shells that would have been dragged up from the seabed, and deposited during the tsunami.
Even though this region isn't marked as one of the areas of Greece susceptible to tsunami, seismic modeling has shown that if there were an earthquake, such a wave could result. Since it's a densely populated area, the effects could be deadly, causing the authors to ask that the region be included in disaster planning.