Wired points out that today is the anniversary of the first recorded sighting of what is known as the Loch Ness Monster. A 7th century account tells the story of how Columba (later St. Columba) saved a man from a monster in the water. From Wikipedia:
...(He) raised his holy hand, while all the rest, brethren as well as strangers, were stupefied with terror, and, invoking the name of God, formed the saving sign of the cross in the air, and commanded the ferocious monster, saying, "Thou shalt go no further, nor touch the man; go back with all speed." Then at the voice of the saint, the monster was terrified, and fled more quickly than if it had been pulled back with ropes, though it had just got so near to Lugne, as he swam, that there was not more than the length of a spear-staff between the man and the beast. Then the brethren seeing that the monster had gone back, and that their comrade Lugne returned to them in the boat safe and sound, were struck with admiration, and gave glory to God in the blessed man. And even the barbarous heathens, who were present, were forced by the greatness of this miracle, which they had seen, to magnify the God of the Christians.
Other recorded sightings began in the sixteenth century, but most have been in the twentieth century, probably because of photography and improved communication. With so many legends and sightings, why is Nessie still a mystery? It's the nature of Loch Ness itself. The lake goes down 754 feet at the deepest point, making thorough searching extremely difficult. It is surrounded by peat, which brings down underwater visibility, and is interconected with other Scottish bodies of water. When you consider what could be hiding in the depths, do you really want to go down and look?