At this point it is pretty clear that the Loch Ness Monster (Nessie) does not exist. I know, logically it is impossible to prove a negative, so if we want to be technical we can say that the probability of a large creature similar to that believed to be Nessie approaches zero. The original 1934 photograph that created the Nessie phenomenon is a confessed hoax. We have 89 years of exploration, including countless visitors hoping to get a glimpse of the creature, camera in hand, and sonar surveys, submarine explorations, multiple webcams, and most recently a DNA survey. The DNA survey is perhaps the most conclusive, because it captures the DNA signature of everything living in the lake. There is no evidence of anything that can be a giant reptile.

If Nessie does not exist, then what have people been seeing all these years? I don’t think we need a concrete explanation for every single sighting. Hard evidence is one thing, but just eyewitness testimony is not really evidence. It is also well known that people misperceive things, confabulate, and are strongly influenced by expectations and desires. In short, if you look hard enough for the Loch Ness Monster, eventually you will see something and convince yourself that you saw the Loch Ness Monster. Even still, it is interesting to hypothesize about what phenomena might trigger alleged sightings, and not just of Nessie, but other lake monsters and cryptids.

One hypothesis is that some eye witnesses may have been seeing other aquatic creatures that might swim along the surface or breech. This includes large alligators, seals, groups of otters, and large fish such as sturgeon. The DNA evidence also ruled out these creatures for the Loch Ness (although they are still candidates for some other lake monsters). DNA, however, did raise the possibility that there are giant eels living in the Loch. Could an unusually giant eel have been mistaken for the neck or tail of Nessie?

A recent paper tests that hypothesis with some statistics. They used data for the European eel (Anguilla anguilla) to calculate likely size distributions. Eels grow throughout their lifespan, and can live over a hundred years. However, their growth is not linear, as it slows down as they age. They calculate that a sighting of a 1 meter long eel in Loch Ness has a probability of 1 in 50,000. Given the size of the lake and the fish stock, they conclude that such sightings are reasonable. So if you think a 1 meter eel could be mistaken for Nessie, it is a reasonable candidate, at least for some of the sightings over the years.

However, if you think a larger eel, say 6 meters in length, is required, they calculate the probability of such a creature in the Loch being “essentially zero”. The larger the eel, the more likely it is that someone would mistake it for the mythical beast, but the less likely that it exists. So unusually large eels are not a likely hypothesis. However, it still leaves open the possibility that someone saw a smaller eel and grossly misjudged its distance and therefore size, or thought they were seeing the tip of Nessie’s tail. A group of eels swimming together may also be confused for a single serpentine creature.

We don’t, however, need one explanation for all sightings. A floating log could be the explanation in some cases, or just a large shadow. One thing we can say for sure is that the photographic and video evidence is not definitive. If it were, you would have seen it. An unambiguous picture or video of a large plesiosaur-like creature in Loch Ness (or any large lake) would be famous. You would have already seen it hundreds of times. It would garner genuine scientific interest, and would be the headliner of every documentary on Nessie or cryptids.

Instead, if you search for the best evidence out there, you see photos and videos of indistinct objects, black-on-black mostly, on the surface of the water. They are tantalizing, at that perfect distance and resolution to be provocative without being actual evidence of anything. They are the Nessie version of blobsquatch, or of the latest UFO flap. The inability to definitively ID the object in the image is the phenomenon. If it were a clear photo, it wouldn’t be interesting.

Chasing down the kinds of things that get mistaken for bigfoot, flying saucers, and Nessie is interesting, and useful. But again, we don’t have to positively identify every blob in order to conclude that these things likely don’t exist. The whole point of this kind of evidence is that they are difficult to ID.

We can also use probability to address the question of how likely each of these alleged phenomena are. We can go back to the 1930’s, 40’s, or 60’s and ask the question – if Nessie, flying saucers, or bigfoot does exist, what is the probability that we will find some definitive evidence over the next 60-90 years? We can also ask – with an exponential increase in the availability of cameras and videos, what is the probability of getting a clear and definitive photo of the alleged phenomenon? Shouldn’t the overall quality of the best evidence increase over time?

Of course, for each of these phenomena, the quality of the evidence has not improved. The evidence is still indistinct – because that is inherent to the nature of the evidence. The Loch Ness webcams alone should be compelling evidence. We have the difference between people occasionally viewing the lock, occasionally with camera or video in hand. This increased over the years with the popularity of the legend. But now with webcams we have multiple 24/7 surveillance of the Loch. This has dramatically increased the probability of capturing something definitive – and we still have nothing.

The simplest explanation for this lack of definitive evidence over nine decades, with increasing attention and digital surveillance, DNA evidence, and multiple explorations, is that Nessie does not exist.