In a word: extraordinary. In 524 BC, when Pythagorio (then called Samos) was the island’s capital and a bustling metropolis of 80,000, securing sources for drinking water became crucial. To solve the problem, ruler Polycrates ordered labourers to dig into a mountainside according to the exacting plan of his ingenious engineer, Evpalinos. Many workers died during the dangerous dig, but the result was the 1034m-long Evpalinos Tunnel. In medieval times, locals used it to hide from pirates.
The Evpalinos Tunnel is actually two tunnels: a service tunnel and a lower water conduit visible from the walkway. You enter the tunnel on narrow stairs, and it's single file from there. The first few metres are pretty tight. There are three options: a 185m section (and return), a longer version and the full 1km (you can walk back outside); guided tours take place every 20 minutes from 8.40am.
Located just north of Pythagorio.