A LOST tribe living in the isolation of an American backwater and a bridge where dogs seemingly go to die - these are just some of the mysteries that are still leaving anthropologists and animal experts baffled.

A far cry from the well-known tales of the Loch Ness Monster and Jack the Ripper, these unknown, unsolved mysteries have flown under the radar.

And they still keep the world wondering as we try to solve their never-ending puzzles.

AMERICA'S LOST TRIBE

Who were the Melungeons? For centuries, these mysterious residents lived in isolation in the central Appalachian Mountains, almost invisible to the American mainstream.

Considered one of the world's greatest anthropological mysteries, the Melungeons lived off the beaten track, hidden away on inaccessible mountain ridges.

Traditionally dark-featured and visibly different from their white, black and Native American neighbours, their ethnic origin has been a source of debate for centuries. Over the years, they've been dubbed American gypsies, Portuguese, descendants of the "lost colony" of Roanoke, and members of a wayward Israeli tribe.

The term Melungeon was traditionally considered an insult and Melungeons were the victims of legal discrimination and racial prejudice well into the 20th century.

It wasn't until the 1960s, when other racial groups found a new pride in their identity, that the Melungeon revisited their own. Rather than reject the term - which had been used against them - they reclaimed it.

Researchers are still trying to crack the Melungeons' DNA code using DNA patterns, oral histories and court records.

THE OVERTOUN BRIDGE

The Overtoun Bridge in Dumbarton, Scotland, has become famous for the number of unexplained instances in which dogs have leapt to their deaths.

The first incident was recorded in the 1950s when it was noticed that dogs would suddenly and unexpectedly jump off the bridge and fall to their deaths. What makes it even more mysterious is that many of the dogs jump from the same side and from almost the same spot: between the final two parapets on the right-hand side of the bridge.

In the past 50 years, some 50 dogs have leapt to their deaths from the bridge.

During one six-month period in 2006, five dogs jumped to their deaths.

The dogs affected are mainly long-nosed breeds: labradors, collies and retrievers.

Dorren Graham, of the Scottish Society for the Prevention of Cruelty to Animals told the Daily Mail the phenomenon was a "heartbreaking mystery".

"There are lots of owners whose dogs have died and who are trying to find out why they jumped."

THE UNCRACKABLE CODE

Carved on an 18th century monument in the grounds of a stately English home, the Shugborough inscription is a cryptic sequence of letters that still has historians baffled.

The reason for the inclusion of the letters DOUOSVAVVM on the Shepherd's Monument at Shugborough Hall is a mystery.

The code - which left even Charles Dickens and Charles Darwin stumped - has been called one of the world's top uncracked ciphertexts.

The carver's identity is a mystery but some have speculated that it could be clue left behind by the Knights Templar about the whereabouts of the Holy Grail.

THE CIRCLEVILLE LETTER WRITER

It looks like any other quaint all-American town. But in 1976, residents of Circleville, Ohio, began receiving mysterious, harassing letters. Thousands of letters, written in block letters were sent to city officials and citizens.

The bizarre campaign turned deadly when one recipient, a school bus driver, received a letter accusing her of having an extra-marital affair.

When her husband drove off to confront the writer after receiving a phone call, he was killed in an unexplained crash.

The woman's brother-in-law, thought to be the letter writer, was later convicted of her attempted murder. However, while he was in jail the letters continued despite him being in solitary confinement and the identity of the writer was never confirmed.

THE ZODIAC KILLER

In the late 60s and early 70s, the Zodiac Killer terrorised northern California, murdering four men and three women. The killer sent letters containing four cryptograms to the local media. Of the four cryptograms sent, only one has been definitively solved. Despite several people coming forward over the years claiming to know the identity of the killer, the murderer's identity is still unknown.

THE ANGEL OF MONS

One of the legends of World War I, the Angel of Mons supposedly safeguarded members of the British Army in a case of divine intervention.

In April 1915, an account was published in the British Spiritualist magazine describing how soldiers had visions of a supernatural force that intervened to help the British at the decisive moment of battle.

This led to other accounts of a strange luminous cloud descending but the most popular version came to be an angelic warrior.

The supernatural visions bear strong similarities to those in a short story by Arthur Machen, published in the Evening Post in September 1914.

THE WOW! SIGNAL

In 1977, American researcher Jerry Ehman was scanning radio waves from deep space when he saw his measurements spike.

The signal lasted for 72 seconds and appeared to have been transmitted from a place no human has gone before: in the constellation Sagittarius near a star called Tau Sagittarii, 120 light-years away.

Ehman wrote the words "Wow!" on the printout of the signal.

All attempts to locate the signal again failed, leading to much mystery about its meaning.

THE CARNAC STONES

And you wonder how Stonehenge was built? On the coast of Brittany in northwestern France are over 3000 megalithic standing stones arranged in perfect lines and spread out over 12 kilometres. How they got there in the first place is a mystery and the identity of the Neolithic people who built them remains unknown.