Two crews dug a tunnel through a mountain from opposite ends — and met inside
In the mid-6th century BCE, the engineer Eupalinos drove an aqueduct straight through Mount Kastro on the island of Samos. Two teams began on opposite sides of the mountain and dug toward each other for more than a kilometer underground, aiming by geometry alone — and met near the middle. Herodotus singled it out as one of the three greatest engineering works in all of Greece, and the tunnel still stands today.
How we know
Tunnel of Eupalinos, Samos, dug through Mount Kastro under the tyrant Polycrates, mid-6th century BCE (start c.550–530 BCE). Length ~1,036 m, excavated from both ends and joined near the middle. Herodotus (Histories 3.60) names it among the three greatest works in any Greek land. It survives as part of the UNESCO 'Pythagoreion and Heraion of Samos'.
One of many fact-checked wonders across eight traditions at The Spring of Wisdom.