Tunnel of Eupalinos

The Tunnel of Eupalinos or Eupalinian aqueduct is a tunnel of 1,036 m length running through Mount Kastro in Samos, Greece, built in the 6th century BC to serve as an aqueduct. The tunnel is the second known tunnel in history which was excavated from both ends, and the first with a geometry-based approach in doing so. Today it is a popular tourist attraction. The tunnel is inscribed on the UNESCO World Heritage List along with the nearby Pythagoreion and Heraion of Samos, and it was designated as an International Historic Civil Engineering Landmark in 2017.

Source: Wikipedia