Bigfoot and the Loch Ness monster are two creatures that some people believe in. To see what it takes to confirm a large animal, it helps to start with some that were confirmed. For a long time the giant squid was known only from sea-monster stories, but it is a living animal of the deep ocean. Scientists obtained a specimen when a Natural History Museum giant squid was caught by accident in a trawler in 2004 and frozen right away, so that DNA samples could be taken from it. In 2013 those samples helped show that giant squid are probably one species, though some scientists still argue about it. Seeing the animal alive took longer: the first video of a live adult giant squid in the wild was shot in 2012 near Chichi Island, Japan, by a submersible that followed the animal about 900 meters down. A submersible is a small vehicle built to work deep under water. A specimen is a sample of an animal that is kept so that investigators can examine it directly, and with a specimen in hand, scientists no longer have to rely on stories alone.

Other large animals have also turned out to exist after science had not known about them. The okapi, a rainforest relative of the giraffe, was unknown to science until 1901. The coelacanth, a fish once thought extinct for millions of years, was found alive in 1938. The saola, a large forest animal, was found by scientists in 1992 in Vietnam, near the Laos border, and it was the first large mammal new to science in more than 50 years. It was first known from a skull with long, straight horns that scientists found in a hunter's home. Each case shows that a species can be missing from the records of science and still be living in the world. Scientists use the word species for one particular kind of living thing, and a species that is new to science is one that no scientist had formally described before.

Scientists use camera traps, which are cameras with motion sensors that take pictures by themselves, to see shy animals. By 2011 the Smithsonian had gathered more than 202,000 camera-trap photos of over 200 kinds of mammals and birds into one website. In 2013 a camera in Vietnam caught a saola, the first time one had been caught on camera there in 15 years. A camera trap does not need a person to stand nearby, so it can record an animal that might keep away from people. The motion sensor starts the camera whenever something moves in front of it, which means that the photographs record whatever actually passes the lens, whether or not anyone expected it to.

Some people say a big, hairy creature called Bigfoot lives in forests in the northwest of North America. It is said to walk on two feet, to be taller than a grown-up and to be covered in long, dark hair. J.W. Burns was the first to write about Sasquatch, collecting accounts from people in his community for a 1929 Maclean's magazine story, and that article was published on April Fool's Day, which led many readers to dismiss it as a prank. John Green kept a database of 1,340 Sasquatch or Bigfoot sightings in North America, from the early 19th century to 1995. In all that time, no body or bones of Bigfoot have ever been found. A sighting is a report that a person saw something, and a database is an organized collection of such reports, so the collection tells us what people said they saw rather than what has been demonstrated.

Footprints are another kind of trace that people have offered as evidence. In 1958 a worker in California found huge footprints near a road being built, and the prints were nearly 16 inches long. That year a California newspaper, the Humboldt Times, started calling the maker of the footprints "Bigfoot." In 2002, after Ray Wallace died, his family said he had made the 1958 footprints with carved wooden feet. Plaster casts of tracks said to be Sasquatch's are often offered as proof, but whether the tracks are genuine is highly disputed. A track is a mark and not an animal, which is why prints and casts can be argued about. A cast is a plaster copy of a print, and a hoax is a trick that is meant to fool people, which is the way that the family's account describes the 1958 prints.

People have offered many ideas about what an unknown ape-like animal might be. Ideas have ranged from Neanderthals to extinct apes such as Gigantopithecus, and Boris Porshnev, a Soviet scientist, suggested that Sasquatch and a Siberian counterpart might be survivors of Neanderthals. Most scientists do not accept that the creature exists, and no bodies or recent fossils of such creatures have been authenticated. A different explanation involves an animal that is already well known to science. Researchers plotted Bigfoot sightings in a 2009 study and found they roughly matched the habitat of the American black bear, and some scientists think people who report Bigfoot may be seeing bears, which can stand up on two legs. A habitat is the kind of place where an animal lives, and a match between where sightings occurred and where bears live is a clue, though not a proof, about which animal people were seeing.

The Oxford team said science should neither reject nor accept a claim without examining the evidence, so it ran a systematic survey of hair samples. For the Oxford study 57 hair samples came in. A plant sample and a glass-fiber sample were removed, and 37 were chosen for DNA tests by where they came from or their history. The hairs were thoroughly cleaned to remove surface contamination before testing, and the authors say human contamination often confounds the analysis of old material. Only one hair gave human DNA, and the team said this showed its cleaning had worked. Almost all of the hairs tested came from known animals such as bears, horses, cows, raccoons and sheep. Professor Sykes said that tests showing hairs came from known animals do not mean the next one won't be something new. Cleaning matters because a hair that carries human DNA from handling would say something about the person, and nothing about the animal it came from.

Samples said to come from the Yeti came from the Himalayas, and the results changed as the work was checked. Two Himalayan hairs in 2014 matched DNA from an ancient polar bear. The authors called the result preliminary because the matching stretch of DNA was only 104 base pairs long. A 2017 survey of bears of the Tibetan Plateau and Himalaya said the earlier polar-bear match had been questioned, and it traced the yeti samples to local, living bears. In 2017 a team led by Charlotte Lindqvist found that eight hairs said to be from the Yeti came from bears of the Himalayas, and a ninth sample, a tooth, was from a dog. Preliminary means early and not yet settled, and the later survey is a good example of how a first result can be checked by other researchers who examine more animals from the same region.

Loch Ness is a very big lake in Scotland, and some people believe a big animal lives there and call it Nessie. A large animal living in the loch would be expected to leave traces behind it, and yet, in spite of many investigations, no bones, bodies, feeding signs or droppings have been found that would support a monster in Loch Ness. Objects have been found in the loch, but they were not animals. 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 shows what people imagined, and it is not the kind of body that could prove a species. The prop is a useful reminder that an object found in the water has to be examined before anyone decides what it is, because a film prop and a living creature are two very different things.

Searchers have also used sound. From 1958, four sonar expeditions at Loch Ness were launched, first by the BBC and then by Oxford, Cambridge and the University of Birmingham. Sonar uses sound to find things under water. In 1987 Operation Deepscan swept Loch Ness with a fleet of sonar boats, and 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. Because the unidentified objects could not be found again, none of them could be examined any further, and an object that cannot be re-examined cannot be identified with confidence.

A hoax can hurt honest inquiry. In 1933 footprints found at Loch Ness were sent to the Natural History Museum in London, where experts said they were made with a hippo-foot umbrella stand or ashtray. After the 1933 hippo-foot tracks, the incident tainted the monster's image and discouraged serious investigation of Loch Ness. Detective work later helped confirm another hoax, the Surgeon's Photograph, a photo that in 1934 seemed to show a long neck rising out of the loch. A 1975 newspaper clip said the photo showed Loch Ness scenery, and the full, uncropped photo, published only once in 1934, did show far shoreline. Because the evidence was a photograph, a newspaper clip and a full print of the same picture, investigators could compare them directly, and that comparison helped to confirm the hoax.

A newer way to look for animals reads what they leave behind, because 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. In another place, a 2018 eDNA study of seawater near New Caledonia found six shark species that baited cameras and long-term observation had missed. Because it detects traces rather than animals, this approach can reveal species that other methods have missed, as the shark study showed, and it does not depend on anyone catching or seeing the animal.

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. Eel DNA turned up in nearly every sample, so there are a lot of eels there, and finding it was not a surprise to the team, since eels were known to live there. The eDNA survey found no shark, catfish or sturgeon DNA in Loch Ness, and eel DNA does not tell how big the eels are, so the team could not rule out giant eels or say that they exist. The survey also aimed to show the power of the method, and its leader, Neil Gemmell, said the monster was "the bait on the very big science hook." Taken together, these results describe what the survey could detect and what it could not, and they show why the eel question stays open.

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. 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. Other scientists say a few big mammals, fish or reptiles may still await discovery in places like the forests of New Guinea or Southeast Asia, or in deep-sea basins. The giant squid and the saola each came with something that researchers could examine directly, a specimen in one case and a skull in the other.

Words to know

• coelacanth: ancient fish
• submersible: deep-sea vehicle
• preliminary: early, uncertain
• contamination: unwanted traces
• hoax: a trick