The way Barcelona's ancient synagogue was rediscovered is a genuine detective story. Beginning in 1987, a historian named Jaume Riera y Sans used 600-year old records to reconstruct the route of a medieval tax collector in El Call, the Jewish ghetto. Following the route, he identified the site of the synagogue, in an old building that canted outwards slightly in order to have one wall that faced Jerusalem.
The building was purchased and cleared, and its origins were found to date back to the 2nd or 3rd century AD. Today, with help from Jews around the world, it was restored in 2002 as a synagogue —the oldest one in Europe, although it no longer has a regular congregation (there are two modern ones serving the city's 3500 Jews), The Associació Call de Barcelona maintains it, and at times they also offer guided tours of the neighbourhood, including a visit to the 13th-century mikveh.
Image by José Luis Filpo Cabana, Creative Commons License