Built in the 6th century BC to serve as an aqueduct to supply water to the ancient town of Samos, now known as Pythagoreio, the Tunnel of Eupalinos (or Eupalinian aqueduct) spans an impressive length of 1,036 m. It has been estimated that the tunnel took a decade for its completion. After thousands of years of use, it fell into disuse during the Byzantine period. In fact, the location of the tunnel was lost for a while. While many searched for it, the discovery was made by a French archaeologist, who found the first 400m of the aqueduct. After its rediscovery in 1882, the tunnel was designated as a UNESCO World Heritage Site in 1992.

A popular tourist attraction, see the bored tunnel which is the highlight of this monumental aqueduct. Once used as a refuge for locals during pirate attacks, look out for the ancient fortress walls at its southern entrance portal. It is also the second known tunnel in history to have been excavated from both ends. A must-see while in Samos!