Considered by Herodotus as the world's Eighth Wonder, this famed underground aqueduct, the To Efpalinio Hydragogeio (or Efpalinio tunnel), was completed in 524 BC with archaic tools and without measuring instruments. 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 also used as a hiding place during pirate raids in the 7th century. Today the tunnel is exclusively a tourist attraction, and though some spaces are tight and slippery, you can walk the first 1,000 feet—also a wonderful way to enjoy natural coolness on swelteringly hot days. Unfortunately the tunnel is closed for rennovations until April 2016, but once these are complete visitors will be able to traverse the whole tunnel.