Humanity has launched satellites into outer space and submarines to the ocean floor, but the ground below Earth's surface remains shrouded in mystery. Are there profound secrets buried beneath layers of rock, or hidden in vast caves? Can the subterranean world below shed light on humanity's past, or even its future?

A look at whether time travel has already occurred, on purpose or by accident, and scientists who believe it is theoretically possible.

Scattered across the globe lie mysterious ruins that are buried in sand-swept deserts, perched atop towering mountains, and shrouded in the heart of dense jungles. These age-old structures stand as silent sentinels from the past, their true history seemingly erased by time. Can modern archaeology unravel the enigma of how-and why-these wonders were built?

During the Middle Ages, the Vikings were infamous for being a bloodthirsty band of seafaring warriors; historians are learning surprising new things about this Norse society.

There are many bizarre tales involving U.S. presidents that defy explanation and are still shrouded in mystery.

There are many puzzling artifacts from around the world that challenge everything known about the technological capabilities of distant ancestors; some ancient objects are so mysterious they've spawned endless debate and theories.

There's nothing more fascinating than a code that can't be cracked. For centuries, mankind has devised ingenious ways to hide valuable information using everything from enigmatic puzzles, to complex ciphers and secret symbols. If history's unbreakable codes can be deciphered, could we unlock answers to some of the greatest mysteries of both the past and present?

During World War II, a number of persistent mysteries were born including unexplained stories about everything from lost gold, to secret societies and highly advanced technology.

Since the dawn of humanity, tales of monstrous creatures lurking in the shadows of our world have sparked both fascination and fear. Today, we often dismiss these accounts as the stuff of myth and legend, but are we overlooking a potentially terrifying reality? With scientists discovering as many as 18,000 new animal species every year, isn't it conceivable that there could be real-life, flesh and blood monsters living among us?