The Euripus is the slimmest gap between Euboea and the Greek mainland, narrow enough that the two shores feel close enough to shout across. Through that gap the Aegean squeezes and surges, and the result is one of the most famous tidal puzzles in the world. The currents can run hard, and they switch direction on a rough six-hour rhythm before tearing into whirlpools that local boatmen learned to read generations ago. Modern oceanographers now explain the basic pattern as a difference in water level building at each end of the channel, but the irregular reversals still resist a tidy answer. Long before anyone measured them, the sailors of Chalkida simply waited for the water to choose its hour, then slipped through. It is the kind of place that rewards patience and punishes hurry.

Chalkida took its name from the Greek word for copper and bronze, and metalwork made it rich. In the eighth and seventh centuries BC, its people poured outward across the Mediterranean, founding some thirty settlements on the peninsula still called Chalcidice and seeding cities across Sicily and southern Italy. Naxos, Rhegion, Zankle, Cumae and the trading island of Pithecusae all traced their roots to this ambitious port. The colonists carried more than pottery and purple dye. They carried their alphabet west, where the Etruscans borrowed it and the Romans inherited it, so that letters like C, D, F, P, R, S and X descend from forms first written here. From this strait, in a real sense, came the shapes of the words you are reading now.

Whoever held Chalkida held a doorway into central Greece, and for two millennia armies kept arriving to claim it. Macedonian kings turned it into a fortress, and Antiochus III and Mithradates VI used it as a springboard for invading the mainland. In the Middle Ages the Venetians ruled it as Negroponte, a condominium shared with the so-called triarchs of Euboea, and built churches and castles whose stones still stand. The Ottomans took the town after a long siege in 1470, raised the Karababa Fortress on the far shore in 1684, and left behind the Emir Zade Mosque. Layered through it all is the story of one of Greece's oldest Jewish communities, the Romaniotes, whose presence here ended in the Holocaust. A memorial outside the Jewish cemetery keeps their memory.

Bridging the Euripus has been a stubborn human project. The first wooden span went across in 411 BC. Under the emperor Justinian it became a movable structure, and over the centuries it was rebuilt as a wooden swing bridge in 1856, an iron swing bridge in 1896, and the famous "sliding bridge" in 1962, which slips sideways to let ships pass at the channel's tightest point. To the south, the cable-stayed Euripus Bridge opened in 1993, a long modern leap over the same restless water. Below them all, the current keeps turning on its own schedule, indifferent to engineering, just as it was when Aristotle stood somewhere near here and tried, and failed, to understand it.