At last count, the world contained about 7.4 billion people. That's quite a lot already, and it's impossible to say exactly how many have lived before us. With so many people in total, it's no wonder that countless stories have been left untold, objects left buried, and mysteries left unsolved.
And as much as we know about the ancient world, there is still so much more we can't fully comprehend - and perhaps never will - about the distant past.
Symbols. Scrolls. Artifacts. Sometimes it's hard to fathom exactly what else could be out there, or even in our own backyards. Some believe there is ancient technology yet to be discovered, or even entire civilizations left to be found.
Click through below to find what some of our ancestors left behind...
The Devil's Bible
Also referred to as the Codex Gigas, the Devil's Bible is a massive medieval tome, written in the 13th century by a monk who was said to have made a deal with the Devil in order to finish it. According to legend, the monk had been sentenced to be walled-up alive as punishment for breaking his vows. He promised the monastery that if they let him live, he would write a book that would contain all human knowledge - in a single day. To fulfill this promise, the monk sold his soul to Lucifer.
The real basis for this legend is likely the fact that even though the manuscript is extremely long, it appears to have been written by just one person over a relatively short period of time.
Created using more than 160 animal skins, and requiring two people to even lift it, the Codex Gigas contains the complete Latin translation of the Bible, as well as multiple other texts, including works by Hippocrates and Cosmas of Prague - not to mention medical formulas, texts on exorcisms, and a large depiction of the Devil himself.
Twelve pages of the original manuscript are missing today, and what they might have contained remains a mystery - some rumors say those pages contain secret Satanic texts, perhaps even a method for conjuring the Devil himself.
The Nazca Lines
By now, it's safe to say most people know are aware of crop circles: sprawling, circular patterns often made in the corn fields of farmers throughout the world, and said to be created by none other than extraterrestrials themselves. Possibly as a form of communication, possibly as a warning.
But perhaps the most fascinating of these types of symbols, dubbed geoglyphs, are the Nazca Lines found across the dry, desert plains of southern Peru.
Said to be created by the Nazca culture between 500 BC and 500 AD, there are more than 100 of these massive glyphs, depicting everything from simple lines, to intricate designs of monkeys, humans, fish, jaguars, and birds. And although the symbols are recognizable, the true purpose of these glyphs is unknown. Some say the lines were created as a means of giving incoming sailors directions. Others say they were used for astronomical purposes, as a way to signal the stars.
In any case, their existence has sparked countless theories on our ancestors' methods of communication, whether to one another or to something beyond.
The Underwater Ruins of Japan
Submerged just off the coast of Yonaguni Jima is Japan's own Atlantis, a sprawling underwater city believed to be at least 5,000 years old.
First discovered by a diver in 1995, the city was first believed to be nothing more than a bizarre series of natural rock formations, all of which mysteriously featured perfect right angles and straight lines. Later searches revealed everything from a large stone gateway to carved stairways and streets to vaulting towers.
Believed to be the former city of the Jomon people, experts theorize that the city didn't exactly "fall" into the sea like the fabled Atlantis, but rather became submerged as sea levels rose over thousands of years. As scientists continue to research the underwater site, many believe this particular site could hold key information to other possible underwater cities around the world.
Unlike many other ancient artifacts, there are actually about a hundred of these mysterious objects floating around. Typically made of either stone or bronze, the hollow Roman dodecahedrons have twelve sides, with a small circle on each, along with various pegs jutting out from the connecting corners.
Although they resemble nautical devices, the purpose of these dodecahedrons is unknown. Historians have guessed everything from fortune tellers to candleholders. Some say they were used for astrological or religious means. Experts have dated them as far back as the 2nd and 3rd centuries AD, and suggest they were used to sow winter grains, or even to calibrate water pipes for Roman architects.
In any case, they've remained a mystery ever since their discovery in Wales and in certain parts of Hungary and Spain.
The King List
Going all the way back to the third millennium BC is the Sumerian King List, a listing of all Sumer kings, their respective dynasties, locations, and times in power. While this may not seem like too much of a mystery, it's what is inscribed along with the list of kings that makes it so interesting.
That is, there are mythological elements embedded within it. Along with a who's-who of Sumerians in power, the King List also incorporates events such as the Great Flood and the tales of Gilgamesh, stories that are often referred to as simple fables.
Theories about the King List range from evidence that Old Testament stories were real, to bizarre explanations that the kings listed were actually gods or demi-gods.
Easter Island's Rongorongo Writings
Easter Island has many strange mysteries associated with it, from the large moai statues (once believed to be only heads, they have now been proven to be entire figures) to the theory that the entire island was once used by extraterrestrials as a means to communicate.
But there is still another strange mystery linked to the famous Polynesian lands - an intricate series of glyphs believed to be writing or proto-writing, an earlier form of communication based on symbols rather than characters.
First discovered by Eugene Eyraud in 1864, the glyphs are said to go back to around 1200, and feature imagery similar to that of other religious signs found in the area. To date, scientists have not been able to determine exactly what the glyphs say, or link them to any outside cultures. The symbols were carved on wood using small obsidian flakes or shark teeth.
Assuming that Rongorongo is indeed a form of writing, it would be a fascinating example of an entirely unique language that formed independently by itself on an isolated island.
The Sajama Lines
Similarly to the Nazca Lines, western Bolivia's Sajama lines are a series of massive drawings etched into the earth, numbering into the thousands.
First discovered in 1932 by Aimé Felix Tschiffely, the lines, between 3-10 feet wide individually, total about 8,700 square miles, all connected via an intricate web-like design. Because of this, they are not only the largest archeological site in the Andes, but possibly in the entire world.
But, like the Nazcas, the question remains: what is the purpose behind these mysterious lines? Scientists have deduced they were originally created by the indigenous people who lived near the Sajama volcano, but for reasons unknown.
Some have speculated they were used for religious purposes, but one can't help but wonder, since they share many similarities with the Nazcas in Peru, could there possibly be some kind of connection between the two sites?
The Hellenikon Pyramid
While it's fairly common to label the Egyptian pyramids the stuff of "ancient mysteries," it's something entirely new to discover similarly mysterious structures all around the world. Such is the case with the Pyramids of Argolis, Greece, and their most famous member, the Hellenikon Pyramid.
The true purpose of the Hellenikon remains unknown, although experts have theorized that a battle once took place at the site, and the structure was subsequently built as a monument to those who died - although nothing has been found within the pyramid's walls to provide evidence of an actual "tomb."
While the mystery of what's inside the Hellenikon is certainly intriguing, what truly propels it into the world of ancient mystery is the fact that it was supposedly built in 2720 BC - making it significantly older than any of the Egyptian pyramids.