A church where Catholics worshiped in secret 300 years ago has been discovered in the middle of Dublin, Ireland.

The church ruins were found after a former office building was demolished, and archaeologists have been called in to document the historic church before construction of a new building commences.

The Irish Times described the find as a “large Catholic church” that served thousands of the faithful for more than a century.

“The first ruins to be uncovered were the thick stone walls of a national school which was still standing in the shadow of the office block as late as the mid-1980s,” the Times reported. “But what lay underneath the old school house proved to be of more interest to archaeologists—the ruins of a considerably older structure which once served as a covert church for Catholics living in the south inner city.”

The Catholics were covert because during “Penal times,” the practice of Catholicism was banned. Nevertheless, the church attracted thousands of worshippers, the Times said. The Catholic Encyclopedia says that persecution of Catholics in Ireland began under King Henry VIII, when the local Parliament adopted acts establishing the king’s ecclesiastical supremacy, abolishing the pope’s jurisdiction, and suppressing religious houses.

“There was probably a building used as a chapel from the foundation of St. Andrew’s parish in 1709 and it is depicted on John Rocque’s map [of Dublin] of 1756,” the archaeological report prepared for the developers says.

Archaeological work is expected to last until Christmas.