Discovered by sponge divers in 1901, the Antikythera mechanism is the world’s oldest analog computer. The mechanism was found aboard the Antikythera wreck, a Roman Period shipwreck resting at a depth of 180 feet. Archeologist have dated the wreck to the 1st century BC. In addition to the Antikythera mechanism, wrecked ship has yielded a trove of glassware, coins, and marble statues.
It is believed ancient mariners used the Antikythera mechanism as a navigational instrument to predict astronomical positions. The mechanism shows signs it was fixed in Antiquity, which indicates it was used often. Archeologists believe the mechanism is Greek in origin, and have dated it from anywhere between 205 BC to 100 BC.
http://www.smithsonianmag.com/history/decoding-antikythera-mechanism-first-computer-180953979/
http://gizmodo.com/the-world-s-oldest-computer-may-have-been-used-to-predi-1781749625
The mechanism, which consists of 30 bronze gears, was housed in a 13 by 7 inch wooden box. Archeologist think the mechanism was operated by a hand-turned shaft, which turned the gears. Each complete revolution of the main gear is believed to equal a year. The Antikythera mechanism remains the most complex geared piece of machinery dating from the ancient world.
The Antikythera mechanism, along with other artifacts recovered from the wrecked ship, are located in the National Archaeological Museum located in Athens.