Considered by Herodotus as the world's Eighth Wonder, this famed underground aqueduct was completed in 524 BC with archaic tools and without measuring instruments. The ruler Polycrates, not a man who liked to leave himself vulnerable, ordered the construction of the tunnel to ensure that Samos's water supply could never be cut off during an attack. Efpalinos of Megara, a hydraulics engineer, set perhaps 1,000 slaves into two teams, one digging on each side of Mt. Kastri. Fifteen years later, they met in the middle with just a tiny difference in the elevation between the two halves. The tunnel is about 3,340 feet long, and it remained in use as an aqueduct for almost 1,000 years. More than a mile of (long-gone) ceramic water pipe once filled the space, which was later used as a hiding place during pirate raids. Today the tunnel is exclusively a tourist attraction, and though some spaces are tight and slippery, you can walk part of the length—also a wonderful way to enjoy natural coolness on swelteringly hot days. Though the tunnel has been closed for necessary engineering work, a partial opening is set for 2018. At some point, with ongoing work, it will be possible to traverse the tunnel in its entirety. On a hillside above the tunnel entrance are the scant remains of a Greek and Roman theater, and a wooden platform over the shell is occasionally used for performances.