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. No body or bones of Bigfoot have ever been found. Instead, people who believe in the creature point to other signs, and footprints are one of them. Footprints said to be Bigfoot's are very large, up to about 20 to 24 inches (50 to 60 cm) long. Let's follow them and see where the name came from.

One set of prints is tied to the name Bigfoot. In 1958 a worker in California found huge footprints near a road being built, and the prints were nearly 16 inches long. That same year, a California newspaper, the Humboldt Times, started calling the maker of the footprints "Bigfoot," so the name began as a newspaper's nickname for whatever had made those prints. Bigfoot also goes by another name, Sasquatch, which is a word that comes from a Salish word that means "wild men."

In 2002, after Ray Wallace died, his family said he had made the 1958 footprints with carved wooden feet. A hoax is a trick that is meant to fool people, and his family's account describes the 1958 prints that way. Plaster casts of tracks said to be Sasquatch's are often offered as proof, and a cast is simply a plaster copy of a print. But whether the tracks are genuine is highly disputed, which means that the people who have studied them do not agree with one another.

Footprints are not the only thing people point to when they talk about Bigfoot. In 1967 Roger Patterson and Bob Gimlin made a 59-second film of an ape-like figure walking near Bluff Creek, the same place as the 1958 footprints. It is the most famous film said to show Bigfoot. Even so, no one has verified it as showing an unknown animal. To verify a claim is to show that it is true, and that has not been done for this film.

Far from the forests of North America, another creature has made a mystery of its own. Loch Ness is a very big lake in Scotland. It is 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. Some people believe a big animal lives in Loch Ness. They call it Nessie. People who report Nessie often describe a long neck, or humps that stick up out of the water.

The modern Nessie story began in 1933, after a new road along the loch gave people a clear view of the water. In that same year 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. So at Loch Ness, as in California, a set of prints was questioned, and in both places the question was whether an animal had made them or a person had made them.

Photos have been questioned too. In 1934 a photo seemed to show a long neck rising out of Loch Ness, and it was called the Surgeon's Photograph. It was shown to be a hoax in 1994, about sixty years after it was taken. The hoax was built from a toy submarine with a model head and neck fitted to its conning tower, and the scene was staged on the loch. Most photos said to show Nessie turned out to be fakes, or to show other animals or objects.

If footprints and photos can be fakes, what else could explain a sighting? Many sightings at Loch Ness could be waves, boat wakes, logs, birds, seals, otters or swimming deer. Some scientists think people who report Bigfoot may be seeing bears, which can stand up on two legs. These ideas do not say that anyone is lying. They are guesses about what an honest observer might have seen, which is a different thing from telling a lie.

An animal would leave more than prints. 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. At Loch Ness, no bones, bodies, feeding signs or droppings have been found that would support a monster. 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.

Words to know

• Sasquatch: Bigfoot's other name
• hoax: a trick
• casts: plaster copies
• disputed: argued about
• verify: show true