Book II, Chapter XXVII of the Vita Columbae of St. Adomnán (627/8–704), abbot of Iona (679–704), contains an account which has been interpreted by many to be the first historic reference to the Loch Ness Monster.
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At another time again, when the blessed man was staying for some days in the province of the Picts, he found it necessary to cross the river Ness; and, when he came to the bank thereof, he sees some of the inhabitants burying a poor unfortunate little fellow, whom, as those who were burying him themselves reported, some water monster had a little before snatched at as he was swimming, and bitten with a most savage bite, and whose hapless corpse some men who came in a boat to give assistance, though too late, caught hold of by putting out hooks. The blessed man however, on hearing this, directs that some one of his companions shall swim out and bring to him the cable that is on the other bank, sailing it across. On hearing this direction of the holy and famous man, Lugne Mocumin, obeying without delay, throws off all his clothes except his under-garment, and casts himself into the water. Now the monster, which before was not so much satiated as made eager for prey, was lying hid in the bottom of the river; but perceiving that the water above was disturbed by him who was crossing, suddenly emerged, and, swimming to the man as he was crossing in the middle of the stream, rushed up with a great roar and open mouth. Then the blessed man looked on, while all who were there, as well the heathen as even the brethren, were stricken with very great terror; and, with his holy hand raised on high, he formed the saving sign of the cross in the empty air, invoked the Name of God, and commanded the fierce monster, saying, ‘Think not to go further, nor touch thou the man. Quick! go back!’ Then the beast, on hearing this voice of the Saint, was terrified and ‘fled backward more rapidly than he came, as if dragged by cords, although before it had come so near to Lugne as he swam, that there was not more than the length of one punt-pole between the man and the beast. Then the brethren, seeing that the beast had gone away, and that their comrade Lugne was returned to them safe and sound in the boat, glorified God in the blessed man, greatly marvelling. Moreover also the barbarous heathens who were there present, constrained by the greatness of that miracle, which they themselves had seen, magnified the God of the Christians.