The 40-metre-wide channel between Chalkis and the Greek mainland is the most famous tidal anomaly in the Mediterranean. Stand on the old (sliding) bridge and you can watch the water visibly running south, slack, then north — and back again, sometimes within four hours. Locals will tell you that during the strongest tides the noise of the rushing water carries through the town.
The best spots to watch are the old Negroponte bridge in the centre and the cafés lining the quay on either side. There is no information panel of any merit; just stand, watch, and try Aristotle's question on yourself.