Loch Ness is located on the Great Glen which bisects the Highlands of Scotland. It resembles a rift valley but is in fact a lateral fault.  The  rocks of Scotland, Scandanavia and North America were once one continent, separated by an ocean from the rocks of England and the rest of northern Europe. Around 420 million years ago in a continental collision called the Caledonian Orogen the rocks of  Scotland collided with the rocks of England. Around 40 million years later, the Highlands north of what is now the Great Glen slipped sideways relative to the south side; geologist W Q Kennedy reported matching granites at Strontian with those at Foyers, 65 miles to the east.  (Quarterly Journal of the Geological Society, 1946)

The great Glen stretches between Inverness in the East and Fort William in the West. There are several lochs in the Great Glen (also known as Glen Albyn or Glen Mhor) but Loch Ness is the longest, and the deepest, with a depth variously said to be between 700 and 900 feet (the North Sea's deepest point is about 150 feet). The Bathymetrical Survey of the Fresh-Water Lochs of Scotland, 1897-1909 reports that in 1901 Murray and Pullar using a wire sounder found  the greatest depth of the Loch to be 754 feet or 230 metres. Deeper sonar soundings have not been independently verified. The average temperature of the Loch varies by only one degree Fahrenheit during the year, as the volume of water is so large and there are few shallows. Some areas of surface water may heat up in the summer by several degrees, for example in the shallows of Urquhart Bay, where the Coiltie and Enrick rivers debouch into the Loch. The bottom of the loch is deeply silted.

 The current history of the loch goes back to the end of the last Ice Age roughly 12,000 years ago when the glaciers melted. During the Iron Age, primitive tribes built a crannog near Fort Augustus, which is now called Cherry Island. It was made by driving oak trunks into the relatively shallow waters on the edge of the Loch, which were then infilled with earth. The crannog was a safe place to retreat to in winter when wolves and bears roamed the area. To this date, it is the only island in Loch Ness. When Thomas Telford built the Caledonian Canal to link the North Sea and the western coast of Scotland, in order to minimise the considerable amount of digging involved a weir was constructed at Dochfour which had the effect of raising the level of Loch Ness by about 9 feet, which means that the crannog is rather smaller now than when it was first constructed. 

Legends of a monster in Loch Ness have existed for over a thousand years. In the sixth century a biography called The Life of St. Columba reports that the saint saved a man from being attacked by the Loch Ness monster simply by making the sign of the cross and commanding the beast to retreat. Other sightings before 1933 indicate some sort of large fish. Mrs Aldie Mackay's sighting of 1933, however, saw the name "Loch Ness Monster" used for the first time and the scene was set for Nessie to become the most enduring mystery of recent years.

In July 1933 Mr George Spicer and his wife had a land sighting. They saw 'a most extraordinary form of animal' cross the road in front of their car. They described the creature as having a large body (about 4 feet high and 25 feet long), and long, narrow neck, slightly thicker than an elephant's trunk and as long as the width of the road (about 10-12 feet wide); the neck had a number of undulations in it. They saw no limbs because of a dip in the road obscuring the animal's lower portion. It lurched across the road towards the loch 20 yards away, leaving only a trail of broken undergrowth in its wake.

Over the subsequent decades, there were more unexplained sightings, mainly of a single hump, sometimes described as like an upturned boat. There were also many hoaxes. The issue took on international importance and several groups of scientists have now undertaken research expeditions attempting to prove or disprove the existence of such a monster. In all, thousands of people have claimed to have seen Nessie, though only a few fuzzy and inconclusive  pictures have ever been captured of the putative animal.