I have given a rather lengthy account of the Samians because they achieved the three greatest engineering works of all the Hellenes. First, they dug a tunnel through a 900-foot-high mountain; it is 4,080 feet long and 8 feet high and wide. Another channel, 30 feet deep and 3 feet wide, was dug along the entire length of the tunnel, into which water is sent through pipes directly into the city of Samos from a huge spring. The builder in charge of designing and excavating this tunnel was a Megarian, Eupalinos son of Naustrophos.
(from The Histories by Herodotus, 3.60, translated by Andrea L. Purvis)
There are several online articles about this remarkable achievement and the more I read about it the more fascinated I am. For those so inclined, I highly recommend Tom Apostol's article The Tunnel of Samos which delves into the mathematical problems that had to be solved in order to dig from opposite ends of the mountain.
Dan Hughes' article The Tunnel of Eupalinos (with an appendix by Hans J. Keller) has plenty of pictures of a walk through the tunnel.
Michael Lahanas, at his Hellenic World encyclopeida has plenty more references for those interested.
Great pictures and more detail can be found at this page by Torben Bolhøj.