St. Placidus was a disciple of St. Benedict, to whom he was brought at Sacro Speco as a child and dedicated to God. There occurred an incident of his rescue from drowning when his fellow monk, Maurus, ran across the surface of the lake Subiaco below the monastery and drew Placidus safely to shore. Around 529 he accompanied St. Benedict to Monte Cassino. Nothing is known of his later life, but he seems to play an important role in Benedictine Order.
Fra Filippo Lippi St. Benedict Orders St. Maurus to the Rescue of St. Placidus.
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365 Saints: Your Daily Guide to the Wisdom and Wonder of Their Lives by Woodeene Koenig-Brick (Author). Harper SanFrancisco, 1995.
Lives of the Saints: From Mary and Francis of Assisi to John XXIII and Mother Teresa by Richard McBrien (Author). Harper San Francisco, 2001.