Stoa of Attalos
In the Ancient Agora of Athens, you'll find a behemoth of a building: The Stoa of Attalos. The Stoa was a Hellenistic achievement - made of marble and limestone, it is 377 feet long and two stories high.
The Stoa was originally built between 159 and 138 BC by King Attalos II of Pergamon and was rediscovered by the Greek Archaeological Society in the late 1800s. Reconstruction began in the 1950s. The building now hosts the Museum of the Ancient Agora which houses finds from the site as old as 4000 BC.