Much is known about the cornerstone of St. Patrick’s Cathedral. As the Archdiocese of New York embarks on a five-year, $175 million renovation of what has been described as the nation’s largest Roman Catholic Gothic sanctuary, architects and historians have meticulously reviewed every detail of James Renwick Jr.’s original blueprints.

They have learned that the cornerstone was hand-cut by Cormack McCall, a 22-year-old Irish immigrant. It was laid on Sunday, Aug. 15, 1858, the Feast of the Assumption, by John Hughes, New York’s first archbishop. Two hundred priests and 100 choirboys marched in the formal procession. The throng of onlookers was so thick — 100,000 strong, by one estimate — that all the city’s streetcars were diverted north to accommodate the crowd. Downtown Manhattan was described as “depleted.”

The stone was left open for offerings from the public. It was sealed exactly two years later, on Aug. 15, 1860.

Much has been learned about the cornerstone, except for two salient details: Where it is and when it went missing.