Proving negatives is difficult. To claim something doesn't exist or won't happen is hard to demonstrate.
Proving a positive claim is much easier. All it takes is a single example to demonstrate a positive assertion.
"If I strike a balloon with a pin it will pop" can be easily demonstrated with a balloon and a pin. Indeed, it would be extraordinary result if the balloon didn't pop. (It can be done, but it is a little tricky.)
But proving that a balloon will never pop is much more difficult. Never is a long time.
It might not pop for years and years but there is nothing saying it won't pop the next day or even the next minute. Proving something will never happen is difficult.
With this in mind, cryptozoology finds just enough wiggle room to keep interest alive.
Is there a monster in Loch Ness? There has never been a credible unchallenged sighting and every bit of instrumentation brought to bear on the lake has found no evidence of the monster. But that doesn't mean the Loch Ness monster doesn't exist - just that we have no evidence for it.
Maybe if we look again, we might find it tomorrow. Or, at least, that is the argument fans of the Loch Ness monster use. It is very difficult to prove the monster is not there.
One of the new techniques being employed by environmental scientists in conjunction with biochemists is called environmental DNA.
It turns out we are always shedding DNA in skin cells. Much of the dust in your house, for example, is sluffed off skin. Taking a sample of the dust allows scientists to determine who was in the house based on the DNA present.
The technique has been applied to rivers and lakes. One research project at UNBC examined the DNA content of fish stomachs to assess what they had been eating. DNA is a good marker for organisms in our environment.
Neil Gemmell and his team from the University of Otago in New Zealand decided to investigate the waters of Loch Ness. They were looking for traces of DNA which might indicate the presence of the monster.
They didn't find any. According to Gemmell, they "didn't find any reptiles at all. We tested a variety of ideas about giant sturgeons or catfish that might be here from time to time but we did not find those either."
On the other hand, they did find DNA traces of more than 3,000 species living in or beside Loch Ness. Human DNA was relatively abundant along with domesticated animals such as dogs, cattle, sheep and pigs. There was also DNA from deer, fish, birds and a multitude of micro-organisms. But nothing which could be construed as belonging to Nessie.
The one thing the work did reveal is Loch Ness has a lot of eels.
According to Gemmell "Out of the 250-odd water samples that we took, pretty much every single sample has got eels in it. But are they giant eels? I don't know."
That the Loch Ness monster might be some form of giant eel has been a possibility bandied about for almost 100 years.
The history of a monster in the lake dates back to the sixth century and the legend has remained in place since then. If the creature is a singular holdover from the age of the dinosaur, as has been suggested, then it would have been inhabiting the lake for at least 1,500 years.
In fact, its lineage would need to be almost 60 million years old.
Nessie really came to the fore in the 1930s when a Scottish newspaper reported a sighting followed by a famous photograph a few years later which was printed in a London newspaper. While the photograph has since been shown to be a hoax, the notion that something inhabits the lake has remained.
All sorts of scientific instrumentation has been brought to bear on the issue but no sign of the beast has ever been found. But since environmental DNA doesn't rely on actually observing the creature, it has the advantage of being able to survey the lake in a very different way.
The 250 samples show which animals, plants, and microbial organisms were in the lake during the two week sampling period in June 2018. None of the samples show any sign of Nessie.
One possibility left is the creature is actually a form of giant eel. Eels as long as three metres have been found in European lakes.
It is possible the conditions in the Loch have allowed a sub-species to grow to prodigious lengths. The DNA evidence is silent on the issue.
While it is hard to prove a negative, environmental DNA makes the usual candidates for Nessie unlikely. But it does not rule out the possibility there might be giant eels inhabiting the Loch. That would be a significant discovery.