The mystery of the Loch Ness Monster, or Nessie, is based on a theory that there is a Neo-dinosaur living in the depths of the lake. Since 1933, there have been documented photos and film of a long necked, hump backed sea monster who lurks in the deeps of Loch Ness in Scotland.
To understand whether this beast could exist we must also understand the loch in which it is supposed to live. Loch Ness is 22.5 miles long and about 744 ft deep. Many have gone searching for the monster here with some new technology like sonar, aircraft, and submarines too.
If this creature is roaming the deeps it is logical to assume that there would have to be more than one. A breeding population in the deeps of the loch would be unlikely due to the level of searching which has been done without results.
However, if the loch had an opening to the ocean or to underground caves then this population size would be possible without being found.