Loch Ness is a very big lake in Scotland, about 23 miles long and about 800 feet deep, and it holds more than 7 billion cubic meters (about 245 billion cubic feet) of fresh water, which is more water than any other lake in Britain holds. Some people believe a big animal lives in Loch Ness, and they call it Nessie. Over the years, investigators have tried several different ways of looking inside the lake, from sound waves to cameras to the genetic traces that living things leave behind, and each method has been able to show some things and unable to show others.

The modern Nessie story began in 1933, after a new road along the loch gave people a clear view of the water. People who report Nessie often describe a long neck, or humps that stick up out of the water, and Nessie witnesses number in the thousands: Nicholas Witchell counted more than 4,000 accounts. Many people who reported seeing something at Loch Ness were sober, level-headed people, such as scientists, teachers, police officers and fishermen. Scottish folklore is full of mythical water creatures, and many early Loch Ness reports seem to have been inspired by it. Accounts of this kind give investigators a reason to search the loch, but accounts alone cannot settle what lives in it.

One early search method was sonar, which uses sound to find things under water. From 1958, four sonar expeditions at Loch Ness were launched, first by the BBC and then by Oxford, Cambridge and the University of Birmingham. Underwater cameras were tried as well. In 1975 underwater photos at Loch Ness looked, after computer enhancement, like flippers, but critics asked if the pictures had been altered to improve their appearance. Photographs from the surface had also met doubt, because the 1934 Surgeon's Photograph was shown to be a hoax in 1994, about sixty years after it was taken.

In 1987 Operation Deepscan swept Loch Ness with a fleet of sonar boats. Three times it found objects in deep water that could not be identified or found again. They were probably debris or thermal effects, or possibly a seal. Scientists and TV teams swept Loch Ness with sonar, and searches in 1987 and 2003 found no monster. Since Operation Deepscan in 1987, nothing in the several sonar sweeps of Loch Ness can be confidently read as a giant animal, and each unidentified echo remained only that: an echo that nobody could find a second time.

Searches have also turned up objects of a different kind. In 2016 a robot drone found a 33-foot Nessie film prop on the bottom of Loch Ness, and it had been made for a 1970 movie. A prop is something built for a film, and a model of a monster is not a monster. At the same time, no bones, bodies, feeding signs or droppings have been found that would support a monster in Loch Ness. A species big enough to be seen has to have many animals, enough to breed, and some scientists doubt that such a group could stay hidden through all the searching.

A newer tool does not look at the water at all, because it reads what living things leave in it. Animals and plants leave bits of DNA in the water around them, from shed cells and waste, and scientists can collect it and read it. The eDNA method was devised in the 1990s, and it has proved crucial in tracking the spread of invasive species. The eDNA in the loch samples showed which animals and plants had been in the local environment in the previous 24 to 48 hours, which means it gives a recent snapshot of the loch rather than a record of the distant past.

In 2018 scientists from New Zealand took more than 250 water samples from Loch Ness to find out what lives there. The survey found about 3,000 different DNA sequences, mostly from tiny things such as roundworms, plankton and small crustaceans. Scientists had also tested the water in Loch Ness and found lots of fish and eels. Eel DNA turned up in nearly every Loch Ness water sample, so there are a lot of eels there. Finding eel DNA was not a surprise to the team, since eels were known to live there, and it showed that the method could detect a known inhabitant of the loch.

The survey answered some questions and left others open. It found no plesiosaur DNA in Loch Ness, and no sturgeon either. A plesiosaur was a sea reptile that died out about 65 million years ago, and some people thought Nessie might be one. Eel DNA, however, does not tell how big the eels are, so the 2018 team could not rule out giant eels or say that they exist. A method that reads DNA can show that a kind of animal was present in the water, but it cannot report the size of the animal that left it.

Many sightings at Loch Ness could be waves, boat wakes, logs, birds, seals, otters or swimming deer. A psychological effect called expectant attention makes people's observations fit what they already expect, such as a water monster at Loch Ness. Most scientists do not think there is a monster in the loch. They say this is because the evidence offered has not been persuasive, not because they refuse to look. The searches described here show that they have looked, with sound, cameras and DNA, and the evidence so far has not changed their view.

As of 2019, Gary Campbell, who keeps a register of Nessie sightings, received on average about 10 reports a year of something unexplained in the loch. In the early 21st century the Loch Ness monster was thought to add nearly $80 million a year to Scotland's economy. The leader of the 2018 team, Neil Gemmell, said he did not believe in the monster, but added: "wouldn't it be amazing if I was wrong?" The searchers who came before him used sound and cameras, and his team used water samples.

Words to know

• sonar: sound that finds
• enhancement: digital touching-up
• thermal: about heat
• eDNA: DNA in water
• plesiosaur: old sea reptile