The Euripus Strait is a narrow channel of water separating the Greek island of Euboea in the Aegean Sea from Boeotia on mainland Greece. It was closed by a dike of coarse sediment until around 4000 BC, but it was opened following an earthquake and remained open until 411 BC, when the Euboeans renounced Athenian hegemony and joined the Boeotian League. The strait's principal port in Chalcis on the island of Euboea.