In the mid to late 90s, Athens built extensions to their subway system. As they dug for stations and vent shafts and tunnels they uncovered a massive number of artifacts, walls, wells, streets, water pipes and more -- by one estimate 50,000 objects.
A number of the metro stations in central Athens have archeological exhibits. Let's take a look. Mouse over the station names for more info.
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A major source of information is "Athens: The City Beneath the City: Antiquities from the Metropolitan Railway Excavations." a 2000 publication of the Greek Ministry of Culture.