At the ruins of Kilmacduagh Monastery in Ireland’s County Galway, an unusual parch mark emerged outside a blocked doorway. Before that doorway was walled up, however, the passage, dating to the eleventh or twelfth century, once served as the entrance to the monastery’s cathedral. But in the fifteenth century, a new doorway was added to the monastery, and the old one likely filled in at that time. By then, generations of monks had entered the church through the earlier door, tamping down the earth outside. Now, more than 500 years after the entrance was sealed, grass still dies more quickly here during extreme dry spells, marking the monks’ ancient path.