An enormous explosion shakes a small town in Texas to the core, while a building demolition goes terribly wrong in Philadelphia. Plus, a bridge in Washington collapses and sends cars down into the river.

Ancient Aliens examines 75 million years of the most credible alien evidence here on Earth, from the age of the dinosaurs, to ancient Egypt, to the skies over the western desert in the present day US.

Below each major city lies city upon city - each with its fascinating and unknown history. This series will reveal the technological marvels that allowed the construction of one city upon another - literally.

Unable to find a tunnel at the waterfall, the team is guided by an animal marker to a boulder carved in the shape of a turtle. Following the direction of the turtle head, the team discovers a Golden Lily symbol indicating a secret backdoor entrance.