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, 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
Most people are likely aware of crop circles: sprawling, circular patterns often made in the corn fields of farmers throughout the world, and said to be created by extraterrestrials. They're possibly a form of communication, possibly 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 BCE and 500 CE, there are more than 100 of these massive glyphs, depicting everything from simple lines, to intricate designs of monkeys, humans, fish, jaguars, and birds. 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.
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, and 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 CE, and suggest they were used to sow winter grains, or even to calibrate water pipes for Roman architects.