Eupalinos (Ancient Greek: Εὐπαλῖνος) or Eupalinus of Megara was an ancient Greek engineer who built the Tunnel of Eupalinos on Samos Island in the 6th century BC.
The tunnel, presumably completed between 550 and 530 BC, is the second known tunnel in history which was excavated from both ends and the first with a methodical approach in doing so. Being also the longest tunnel of its time, the Tunnel of Eupalinos is regarded as a major feat of ancient engineering. It was constructed for the tyrant Polycrates of Samos, and was a remarkable 1,036 meters (3,399 ft) long. It brought water to the city, passing through limestone at the base of a hill; this tunnel still exists.
The Greek historian Herodotus describes the tunnel briefly in his Histories (3.60) and calls Eupalinos of Megara its architect:
I have dwelt rather long on the history of the Samians because theirs are the three greatest works (ergasmata) of all the Greeks. One is a tunnel (orygma amphistomon) through the base of a nine hundred foot high mountain. The tunnel's length is seven stades, its height and length (width) both eight feet. Throughout its length another cutting (orygma) has been dug (ororyktai) three feet wide and three feet deep, through which the water flowing in pipes is led into the city from an abundant spring. The builder (architekton) of the tunnel was the Megarian Eupalinus, son of Naustrophus.
Eupalinos is considered the first hydraulic engineer in history whose name has been passed down. Apart from that, though, nothing more is known about him.
A large road tunnel, named after Eupalinos has been recently built under the Geraneia mountains in Corinthia, to facilitate the new expressway connection between Athens and Corinth. Eupalinos tunnel is the longest of three subsequent tunnels of the same width at this expressway.