Terry L.
Google
This 1876 market was to be renovated a few years back, and while going some earthwork to secure the structure, they uncovered the ruins from 14-16th century Barcelona. What followed was a full excavation and preservation of BCNs merchant, craftsman, and artisan quarter.
Entry is free and you can walk along the catwalks, looking down into the past to see how the people lived and worked. A paid tour (€4) offers a guided explanation (in multiple languages), extensive exhibits on artifacts pulled from the rubble, and a walk down onto the actual cobblestone streets.
It's just a block or so from the Picasso and MOCO museums, the Santa Maria del Mar basilica, the zoo/Cuitadella, and surrounded to boutiques and restaurants. Together, it makes a perfect day of exploring BCNs past.