Few people ever knew, but the medieval alleys of the Portuguese city of Porto on the Atlantic coast once provided cover for a persecuted minority at risk of being burnt at the stake.
In the 16th century, a thick-walled granite house that still stands in a row of narrow buildings along a cobbled street held a dangerous secret. At the back, steep steps lead down to a warren of alleys ideal for conspiratorial comings and goings that helped keep an outlawed religious ceremony hidden.
Four centuries later, the secret of the synagogue is out. The mystery began unravelling when Fr Agostinho Jardim Moreira, a Catholic priest, bought the four-storey house for use as an old people's home for his parish. When construction workers told him they had come across a false wall, he told them to pull it down - sensing a hidden tale.
He had studied the city's Jewish history and knew his parish had been a Jewish quarter in the 15th and 16th centuries. He also knew that, after they were forced to convert to Catholicism in 1496, many Jews privately kept their faith and worshipped in secret. "I suspected that false wall was hiding something," said Fr Moreira. "I knew there had to be some kind of Jewish symbol behind it."
A worker's sledgehammer proved his hunch right. Beyond the wall was a room with a medieval holy ark - a nook in the wall of a synagogue where Torah scrolls are kept. Only two other arks from the period have been found in Portugal.
After corroboration by historians, the Portuguese Institute of Architectural Heritage authenticated the house last month as the site of a secret synagogue.
The chance discovery solved an enigma that had baffled historians for years, said Elvira Mea, a lecturer who specialises in Jewish history at the University of Porto. Immanuel Aboab, a 16th-century Jewish scholar, had written that, as a child, he had visited a synagogue in the third house along the street counting from the 14th-century Our Lady of Victory church.
But he didn't specify which side of the street, and archaeological digs had turned up nothing. Then came confirmation of the accuracy of Aboab's text: the house Fr Moreira bought was the third house down on the street the Jewish scholar had described.
Historians had been thrown off by the fact that Aboab never described the synagogue as clandestine. His childhood experiences took place five decades after the forced conversion - at a time when secret Jewish worshippers would be tortured and burnt at the stake if caught - so there was no chance a synagogue could function in the open.
"Everyone assumed Aboab had got his dates mixed up," said Professor Mea. "But it had been preying on my mind and, as soon as I saw the ark, all the pieces fell into place."
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