A mysterious rock bearing a cryptic design was discovered in Roswell, New Mexico in 2004. This piece of stone is unusually smooth and seems to possess unusual magnetic properties.

Ever since Plato first wrote about the mysterious island of Atlantis, it has captured the imagination of adventurers and academics alike. Was Atlantis merely a myth, or could it have actually existed in the remote past?

A top-secret Nazi project - "Die Glocke," was believed to be a time travel device that could soar through the sky and eventually reach the speed of light and beyond. Did Die Glocke actually exist, and could it have been based on alien technology?

The Loch Ness Monster is one of the world's most enduring mysteries. Are these fantastic creatures leftover dinosaurs, mythical animals or something that travelled to Earth millions of years ago?

In 1958, a bulldozer operator in California discovered 16-inch human-like footprints, and the legend of Bigfoot was born. Today, there are hundreds of reported sightings, but nowhere are the stories more prolific than in America's Pacific Northwest.

The mysterious Mediterranean island of Malta lies south of Sicily and is home to megalithic structures built by a mysterious prehistoric society. Who were the people that built these sophisticated structures?

The ancient ruins of Puma Punku have mystified archaeologists ever since their discovery hundreds of years ago. Is it possible that the ruins of Puma Punku offer evidence of advanced technology being used in the ancient past?

The architecture in Washington DC was inspired by the ancient world. But is it possible that the designs and philosophies enshrined in the streets of the US capital were influenced by an otherworldly source?

Giorgio Tsoukalos heads to Peru to investigate whether the strange symbols covering its desert floor are key to proving aliens once visited Earth.

All around the world are examples of physical evidence suggesting humans are not alone in the universe, but has this evidence been left behind for a purpose? Is it a series of clues we are meant to decipher? Giorgio A Tsoukalos investigates.