Loch (lake) Ness is one of a series of interconnected, murky water lakes in Scotland. Quite large and deep, it features exceptionally low water visibility due to a high peat content in the surrounding soil. Rumours of a large monster or animal living in the loch have been circulating for at least several centuries, although to date there has been no substantive evidence to that effect. Local inhabitants still argue strongly for its existence, however skeptics retort that this is probably because the rumours underpin the local tourism industry.
Most accounts of the monster's appearance, including historical ones, indicate a creature with a striking resemblance to the long extinct Plesiosaur. Actual fossil evidence for this prehistoric creature shows it to have been physically large, with a long neck and tiny head, and flippers for propulsion. The alleged connection of this creature with the Loch Ness monster has made it a popular topic in the field of Cryptozoology. However, most scientists agree that the idea that it is a remnant of the prehistoric era is not plausible - there would need to be a breeding colony of such creatures for there to have been any long term survival, and this would result in far more frequent sightings than have actually been reported.
The majority of other theories as to the exact nature of the Loch Ness monster are considerably less sensational. The sighting of disturbances in the water caused by seals, fish, logs, mirages & light distortion, crossing of boat wakes, or unusual wave patterns have all been proposed. Very large sturgeon have been found in inland streams closs to Loch Ness and due to sturgeons' size and unusual apperance, one could easily be mistaken for a monster by someone not familar with it. A recent theory postulates that the 'monster' is actually nothing more than bubbling and disruptions in the water caused by minor volcanic activity at the bottom of the loch. This latter argument is supported (to a minor degree) by the correlation between tectonic motion and reported sightings.
Regardless of whatever is actually in the Loch, the Loch Ness monster is certainly very real with regard to the local economy. Dozens of hotels, boating tour operators and merchants of stuffed animals and related trinkets owe their livelihood to the "existence" of this monster, and hence the legend is likely to endure for quite some time.
A similar creature, "Champ", has reported in Lake Champlain in the United States. Ogopogo in British Columbia's Lake Okanagan is another, and the most well-known example of a multitude of such obscure monsters in Canada -- NaDene Indian mythology is replete with lake creatures, and many of these have been picked up by European-descended settlers.