Sightings of the mysterious animal are said to go back to St. Columba in 582 as reported by Adamnan in The Life of St. Columba, but not in the loch itself (it was in the River Ness), and the report may not have been reliable (Loch Ness Monster-crystalinks.com). Modern sightings begin in 1871 (the 1st modern water-based case; the 1st land-based case was in 1879), but especially since 1933, and according to Roy Mackal, in his book The Monster of Loch Ness, there have been 3000 (strangemag.com). LNIB (Loch Ness Investigation Bureau) has compiled 1000 (loch-ness.org).
Some of the sightings are misidentications of logs or known animals and some are hoaxes, but some are the actual monster. It is described as being about 10-40 ft. in overall length, and usually as having a small, snake-like head (with horns or antennas often mentioned), long neck, bulky body with humps (usually 2 or 3), 4 short, thick legs like an elephant or dinosaur or webbed feet, or flippers, and a long tail (nessy.co.uk). 45% of the reports describe humps, which disappear once the animal straightens out, and 20 % are of a back or body without humps, which is most commonly described as like an upturned boat, but also as similar to an elephant's back or a giant eel (nessy.co.uk). There was at least 1 mass sighting--in '58, a bus-load of people saw a creature with 2 humps, the larger one estimated at 25 ft. long (loch-ness-monster-nessieland).
Several expeditions have been in pursuit of the elusive creature. There have been 21 sonar probes and 1 hydrophone probe (from 1954-2008) (15 tabulated in nessy.co.uk and another 6 sonar probes and 1 hydrophone probe described in Loch Ness Monster-Wikipedia; Monstre du Loch Ness-Wikipédia), 9 getting positive results. These were done by Birmingham U., LNIB, Robert Rines' Academy of Applied Science, the World Book Encyclopedia, Oxford, Cambridge, and the BBC, among others. Here is a summary of the highlights:
'68: LNIB team detects multiple animate objects ascending and descending to the bottom at 10 knots (12 mph or 20 km/h); they were clearly animals and were not air-breathers and probably not fish.
'70: Mackal's Big Expedition hydrophone system detects chirps, knocks, and clicks, the latter 2 indicating echo-location followed by a turbulent swishing indicating a tail and a moving-in for the kill; there is no similarity between these sounds and those of 100s of known aquatic animals.
'87: Operation Deep Scan made sonar contact with a very large object of unusual strength and another large moving object at a depth of 600 ft.
'93: Discovery Loch Ness makes 3 sonar contacts each followed by a powerful wake; also, it is discovered that the fish population is 9 times previous estimates.
The Rines expeditions got positive results but I leave them out as Tony Harmsworth considers them to be improperly done and unreliable at best and makes a good case (loch-ness.org), and he is a believer so has credibility. He himself had a sighting and took some pictures.
And there have been some 30 cases of film evidence (from 1933-2010)(nessy.co.uk; Monstre du Loch Ness-Wikipédia). Peter Scott, British naturalist, and member of the LNIB, named the beast Nessiteras rhombopteryx based on a picture showing a rhomboid fin, taken by 1 of the Rines expeditions, but the picture had been enhanced, and it is unclear if the original really showed such an object, and Harmsworth considers it a fake. None of the motion-pictures or stills are conclusive, and all of the classic ones have turned out to be fakes (loch-ness.org). However, a recent (2016) photo by Ian Bremner is clear and large and does seem to show the classic humps. Skeptics, who are often and erroneously called debunkers, say it is 3 seals, but it doesn't look like that.
The LNPIB (later the LNIB), which operated from '62-'72, was founded by MP David James and Peter Scott. In '69 it had over 1000 members, over half from the UK, and its directors included David James, Peter Scott, and Roy Mackal, among others, and the Chairman was Norman Collins (Loch Ness monster-Wikipedia).
The most popular theory as to the nature of the beast is that it is some sort of plesiosaur, but Dutch zoologist Antoon Oudermans in 1892 suggested that sea serpents were a type of eel and in the '30s he suggested Nessy was of this type. Lt. Cmdr. Rupert Gould (the Case for the Sea Serpent, Loch Ness Monster) suggested something like a long-necked newt, and Mackal gave it an 88% probability on his list. Bernard Heuvelmans has maintained that they are a pinniped, specifically an unknown species of long-necked seal. But Harmsworth rules all of these out except amphibians but says it is most likely a fish, probably a sturgeon, but this does not fit the descriptions.
Heuvelmans (Wikipedia) is the father of cryptozoology and coined the term "cryptozoologie". He was born in France and raised in Belgium, has a PhD in zoology, founded the Centre de Cryptozoologie in '75 in France, was a founder of the ISC (International Society of Cryptozoology; its official organ is the Journal of the International Society of Cryptozoology) in DC, and was its 1st president, and was 1st president of the Center for Fortean Zoology. He wrote 5 books: Sur la piste des bêtes ignorées ('55), Dans le sillage des monstres marins ('58), Le Grand-Serpent-de-Mer ('65), Les derniers dragons d'Afrique ('78), and Les bêtes humaines d'Afrique ('80). The 1st 3 were translated into English. In '99, he donated 50, 000 documents, photos, and specimens to the Lausanne Musée de Zoologie.
The difficulty in detecting the creature is probably because of its small numbers, estimated at about 1 to several dozen or so (Loch Ness Monster-unmuseum), the large size of the lake, and possibly that it burrows into the lake bottom for warmth, and the probes are probably noisy which would scare it away (lochnessmystery.blogspot.ca).
There are over 80 such monsters in an incomplete list on Wikipedia. Here is a tally table:
Australia 2 (Bunyip and Aggy)
Alaska, NZ, Costa Rica, and South America each have 1. The most famous in North America are Ogopogo in Lake Okanagan in BC and Champ in Lake Champlain in NY and Vermont, its northern shore bordering Quebec.
Although the sound detection and photographic and film evidence do not provide overwhelming or conclusive evidence (altho the Bremner picture might be considered as such), they do provide an indication that there is something unusual there, and there are just too many cases of independent, credible, and reliable witnesses describing an unknown animal for the phenomenon to be attributed entirely to hoaxes or misidentifiications, so we can conclude there is an unknown creature in the loch and similar creatures in many other lakes around the world.
More than a Legend Constance Whyte
Encyclopedia of the Loch Ness Monster Paul Harrison
Loch Ness and the Monster Nicholas Witchell
Loch Ness and its Monster J.A. Carruth
There are as well many other cryptic creatures such as hominids: the Sasquatch, Yeti, Agogwe of Tanzania (resembles australopithecines), the Orang Pendek of Sumatra, the Almas of the Pamir Mtns. of Tadjikistan, the Yahoo or Yowie of the Australian Outback; wolfmen or werewolves; the Su or Succarath of the Argentine pampas (possibly a supposedly extinct ground sloth (mylodon)); flying creatures: the Thunderbird of the SW US (which may be a supposedly extinct, giant, flying reptile (a pterodactyl or pterosaur)), the teratorn, the Kongamato of Zambia's Jiundu Swamp (a pterodactyl or giant bat); the Mokele-mbembe (large reptile, possibly a sauropod that escaped extinction) of the Congo's Likouala Swamp; the water-tiger of South America; and the Nandi Bear of the Great Rift Valley in Africa. Recommended books are those by Heuvelmans mentioned above and Alien Animals by Janet and Colin Bord.