How can we know who we are if we don't know where we come from? It is clear from many fragments of evidence, traditions, and lore that we have an incomplete picture of the earliest days of human civilization. It's possible that whole civilizations, some with advanced technology, have come and gone. At the very least, human culture reaches much further back in time than conventional history admits. There are many mysteries in our ancient past, but there may be clues to that past around the world in the form of sunken cities, ancient structures, cryptic hieroglyphics, artwork and more.

Here are 10 of the most intriguing pieces of the puzzle that is our past. They are shrouded in mystery and varying degrees of doubt, but all are nonetheless fascinating.

1. Egyptian Treasures in the Grand Canyon

The April 5, 1909, edition of the Arizona Gazette featured an article entitled "Explorations in Grand Canyon: Remarkable finds indicate ancient people migrated from Orient." According to the article, the expedition was financed by the Smithsonian Institute and discovered artifacts that would, if verified, stand conventional history on its ear. Inside a cavern "hewn in solid rock by human hands" were found tablets bearing hieroglyphics, copper weapons, and statues of Egyptian deities and mummies.

Although highly intriguing, the truth of this story is in doubt simply because the site has never been re-discovered. The Smithsonian disavows all knowledge of the discovery, and several expeditions searching for the cavern have come up empty-handed. Was the article just a hoax?

"While it cannot be discounted that the entire story is an elaborate newspaper hoax," writes researcher/explorer David Hatcher Childress, "the fact that it was on the front page, named the prestigious Smithsonian Institution, and gave a highly detailed story that went on for several pages lends a great deal to its credibility. It is hard to believe such a story could have come out of thin air."

2. Age of the Pyramids and Sphinx

Most Egyptologists believe the Great Sphinx on the Giza plateau is about 4,500 years old. But that number is just that — a belief, a theory, not a fact.

As Robert Bauval says in "The Age of the Sphinx," "there was no inscriptions — not a single one — either carved on a wall or a stela or written on the throngs of papyri" that associates the Sphinx with this time period. So when was it built?

John Anthony West challenged the accepted age of the monument when he noted the vertical weathering on its base, which could only have been caused by long exposure to water in the form of heavy rains. In the middle of the desert? Where did the water come from? It so happens that this area of the world experienced such rains about 10,500 years ago! This would make the Sphinx more than twice its currently accepted age.

Bauval and Graham Hancock have calculated that the Great Pyramid likely dates back to about 10,500 BCE — predating the Egyptian civilization. This raises the questions: who built them and why?

3. Nazca Lines

The famous Nazca lines can be found in a desert about 200 miles south of Lima, Peru. On a plain approximately 37 miles long and one mile wide are etched lines and figures that have puzzled the scientific world since their discovery in the 1930s. The lines run perfectly straight, some parallel to one another, many intersecting, making the lines look from the air like ancient airport runways.

This prompted Erich von Daniken in his book "Chariots of the Gods" to suggest they were actually runways for extraterrestrial craft ... as if they would need runways. More intriguing are the gigantic figures of more than 70 animals carved into the ground — a monkey, a spider, a hummingbird, and others. The puzzle is that these lines and figures are of such a scale that they can only be recognized from a high altitude. (They were rediscovered by accident in the 1930s by an overflying airplane.) So what is their significance?

Some believe they have an astronomical purpose, while others think they served in religious ceremonies. A recent theory suggests the lines lead to sources of precious water. The truth is, no one really knows.

4. Location of Atlantis

There are as many theories as to the true location of Atlantis as there is spam in your email box. The legend of Atlantis comes from Plato, who wrote about the beautiful, technologically advanced, continent-sized island back in 370 BCE, but his description of its location was limited and vague. Many, of course, conclude that Atlantis never really existed and was merely a fable.

Those who think it did exist have sought evidence or at least clues in almost every corner of the globe. Edgar Cayce's famous prophecies said remnants of Atlantis would be found around Bermuda, and in 1969 geometric stone formations were found near Bimini that believers said confirmed Cayce's prediction. Other proposed locations for Atlantis include Antarctica, Mexico, off the coast of England, and possibly even off the coast of Cuba (see below). Writer Alan Alford makes the case that Atlantis was not an island at all, but rather an exploded planet. The controversy and theories will likely continue until someone uncovers a sign saying: "Atlantis, pop. 58,234."

5. Maya Calendar

There has been a lot of hand-wringing over the supposed prophecies of the Maya calendar. More people fear it, perhaps, than feared the ominously predicted catastrophes of the year 2000. All the fretting is based on the finding that the Mayan "Long Count" calendar ends on a date that corresponds to our December 21, 2012. What does this mean? The end of the world through some global cataclysm or war? The beginning of a new era, a new Age for mankind? Such prophecies have a long tradition of not coming to pass. 2012 has obviously come and gone, but some people still think there's something to the prophecy — that 2012 was just the beginning.

6. Japan's Underwater Ruins

Off the southern shore of Okinawa, Japan, under 20 to 100 feet of water lies enigmatic structures that may have been built by some ancient, lost civilization. Skeptics say the large, tiered formations are probably natural in origin. "Then, in late summer of the following year," writes Frank Joseph in an article for Atlantis Rising, "another diver in Okinawa waters was shocked to see a massive arch or gateway of huge stone blocks beautifully fitted together in the manner of prehistoric masonry found among the Inca cities on the other side of the Pacific Ocean, in the Andes Mountains of South America." This seems to confirm that these are manmade ruins.

The architecture includes what appear to be paved streets and crossroads, large altar-like formations, staircases leading to broad plazas and processional ways surmounted by pairs of towering features resembling pylons. If it is a sunken city, it is huge. It's been suggested that it might be the lost civilization of Mu or Lemuria.

7. Voyages to the Americas

We were all taught that Christopher Columbus discovered America; what they meant to teach us, however, was that Columbus began the official European invasion of the Americas. People had "discovered" the continent long before Columbus, of course. What are known as Native Americans arrived here many centuries before Columbus, and there is good evidence that explorers from other civilizations beat Columbus here too. It is widely accepted that Leif Erikson successfully sailed to North America in the year 1000.

Far stranger, artifacts have been found suggesting that ancient cultures explored the continent. Greek and Roman coins and pottery have been found in the U.S. and Mexico; Egyptian statues of Osis and Osiris were found in Mexico, to say nothing of the abovementioned Grand Canyon discovery; ancient Hebrew and Asian artifacts have also been found. The truth is, we know very little about early, far-traveling cultures.

8. Sunken City off Cuba

In May 2001, an exciting discovery was made by Advanced Digital Communications (ADC), a Canadian company that was mapping the ocean bottom of Cuba's territorial waters. Sonar readings revealed something unexpected and quite amazing 2,200 feet below the surface: stones laid out in a geometric pattern that looked very much like the ruins of a city. "What we have here is a mystery," said Paul Weinzweig of ADC. "Nature couldn't have built anything so symmetrical. This isn't natural, but we don't know what it is." A great sunken city? It must be Atlantis, was the immediate suggestion of many enthusiasts.

National Geographic showed a great deal of interest in the site and was involved in subsequent investigations. In 2003, a minisub dove down to explore the structures. Paulina Zelitsky of ADC said they saw a structure that "looks like it could have been a large urban center. However, it would be totally irresponsible to say what it was before we have evidence." Further explorations are forthcoming.

9. Mu or Lemuria

Nearly as famous as Atlantis is the legendary lost world of Mu, sometimes called Lemuria. According to tradition among many Pacific islands, Mu was an Eden-like tropical paradise located somewhere in the Pacific that sunk, along with all of its beautiful inhabitants, thousands of years ago. Like Atlantis, there is ongoing debate as to whether it really existed and, if so, where. Madame Elena Petrovna Blavatsky, the founder of the Theosophy movement in the 1800s, believed it was in the Indian Ocean. The ancient residents of Mu have become a favorite of channelers who bring their enlightened messages to present times.

10. Caribbean Underwater Pyramids

One of the most intriguing tales of the discovery of ruins of a lost civilization is the story of Dr. Ray Brown. In 1970, while diving near the Bari Islands in the Bahamas, Dr. Brown claimed to have come across a pyramid "shining like a mirror" that he estimated was 120 feet tall, although he could see only the top 90 feet. The pyramid had a colored capstone and was surrounded by the ruins of other buildings. Swimming into a chamber, he found a crystal held by two metallic hands. Over the crystal hung a brass rod from the center of the ceiling, at the end of which was a red multifaceted gem of some kind. Brown said he took the crystal, which allegedly has strange, mystical powers.

Brown's story sounds fictitious — it's just too spectacular. But it excites the imagination about all the mysteries of lost worlds awaiting rediscovery.