The Euripus separates the Greek island of Euboea from Boeotia on the mainland, and at Chalcis it pinches down to its narrowest point. Here the water rushes through at up to twelve kilometers an hour, first one way and then the other, changing direction roughly four times a day. The most dangerous moments come at the turn. As the flow slows to reverse, whirlpools spin up in the channel, and a vessel caught at the wrong instant can be spun helplessly. Tides in the eastern Mediterranean are usually so slight as to be irrelevant to sailors, which is exactly what makes the Euripus so startling. The same forces that barely register elsewhere are funneled through this gap and amplified into something fierce.

The strange tide became a famous puzzle of the ancient and modern world alike. A persistent legend holds that Aristotle, who studied on Euboea, was so frustrated by his failure to explain it that the riddle haunted his final days. The science took far longer than anyone expected. In the nineteenth century, the Swiss scholar Francois-Alphonse Forel made progress by studying how layers of water at different temperatures slosh back and forth in an enclosed basin, a phenomenon he named the seiche. But a full explanation arrived only in 1929, when D. Eginitis, director of the Athens Observatory, finally published the solution after centuries of speculation. The name itself carries the mystery: euripos comes from ancient Greek words meaning a strait "where the flux and reflux is violent."

The Euripus has not always been open water. For thousands of years it was blocked by a natural dike of coarse sediment, until roughly six thousand years ago an unrecorded earthquake tore it open and let the sea pour through. From then on the channel became a strategic prize. When Herodotus described the naval battle of Artemisium in 480 BC, he implied that whole fleets of war triremes could pass through the strait. A few decades later, in 411 BC, the people of Euboea did something remarkable: they tried to undo the earthquake. By rebuilding most of the dike, they hoped to reconnect their island to the mainland and slip free of Athenian control. They left only narrow gaps for the tide, which made the current more violent still, and just one passage wide enough for a single ship to thread.

For the rest of antiquity and the Middle Ages, engineers fought a running battle with the strait. The geographer Strabo recorded a bridge across it; the historian Procopius, writing under Emperor Justinian in the sixth century, described two channels, one of them so narrow it could be crossed by a single plank of wood. Over the centuries the place gathered names like silt. The fortress here became Egripos, then, when Latin crusaders seized it in 1205, Negroponte, a name that has nothing to do with any black bridge but came from a Venetian play on the older words. Venetian archives record the daily anxieties of running a port on a current this strong: a reef forming under the bridge in 1408, fears in 1439 that the racing water was scouring away the pilings, and water mills on the narrow channel that the flow sometimes simply tore apart.

The shipping passage we see today was carved slowly out of that history. The Ottoman traveler Evliya Celebi, who visited in 1668, noted that the narrow eastern channel had only been opened enough for a galley to squeeze through in the late sixteenth century, and was still barely wide enough in his own time. By the end of the eighteenth century it had grown close to its modern width. Two road bridges now cross the strait at Chalcis, and the older of them can still slide aside to let ships pass. But for all the engineering, the water beneath them keeps its ancient habit, turning back on itself four times a day, indifferent to the centuries of people who have tried to understand it.