For years, the team has been tracking an elusive prey only seen at night--ships, shaped like triangles, impossibly huge, and videotaped over cities around the world.

It could be one of the darkest and most sinister secrets in the entire field of UFO investigation: human-alien genetic experiments.

They are described by hundreds of witnesses as grey-skinned beings: no ears, no nose and bulbous black eyes. Are these so called Greys visitors from another planet?

On November 11th 1987, the small town of Gulf Breeze, Florida is inundated with UFO sightings. More than 200 witnesses see strange, unexplained lights in the sky.

It has long been suspected that Nazi leaders had more than a passing interesting in the occult and UFOs.

It's a disturbing pattern that's been going on for the past 40 years--thousands of cows seemingly slaughtered in the same strange, meticulous way on ranches around the world.

From microscopic particles, to large pieces of unknown metal--UFO trace cases offer some of the most convincing evidence that UFOs may have been visiting earth for centuries.

Everyone's heard of Area 51--Groom Lake, Nevada. However, 2,500 miles away there's an underwater facility that's also highly secretive.

Ever since Roswell, observers have noted that UFOs allegedly appear in locations that are already full of secrets and mystery: namely, weapons labs. The question is, why?

There's a growing phenomenon of alleged UFO sightings known as Orbs. Witnesses who see the orbs also report having dreams, premonitions, and a deep fear.

For decades, it has been believed that the United States military have had unexplained encounters with the UFO phenomena. Our team sets out to try and uncover the armed forces' first response to UFO encounters. They investigate a claim that in Long Beach, California that multiple UFOs were seen and pursued by military helicopters. They revisit the area of the alleged 2008 UFO crash in Needles, California. Here, it is believed the military presence has been beefed up as the number of UFO sighting has grown.

UFOs are the biggest mystery of our time and somebody - or something - seems determined to keep it that way.

Area 51 is considered the ultimate top secret facility, one that UFO investigators believe houses an extraterrestrial secret. But is there an even more secretive facility?