Are we truly alone in the Universe? Ancient Astronaut theorists propose that descriptions of sky gods and star beings might reflect real encounters with extraterrestrial visitors in the remote past. Could this be true? Follow experts Giorgio Tsoukalos, William Henry, and Jason Martell as they sift through the series archives of "Ancient Aliens" to uncover in-depth descriptions and portrayals of these entities from Egyptian, Sumerian, Chinese, Mesoamerican, and Native American traditions. Could the evidence reveal these beings to be… ancient astronauts?

Beneath the surface of China lies the Longyou Grottoes, a breathtaking subterranean marvel featuring grand staircases, expansive hallways, and colossal ten-story columns carved from rock–believed to be over 15,000 years old. This remarkable discovery mirrors Derinkuyu, an enigmatic underground city found in Turkey. Meanwhile, in the American Southwest, secret tunnels linked to Native American lore of the Ant and Snake People are rumored to house covert government UFO programs. And in Egypt and Ecuador, libraries from a lost civilization have long been speculated to be concealed in underground caverns. Might these locations, and others, validate ancient legends of underground realms inhabited by extraterrestrial beings? And if so, what other otherworldly secrets might lie beneath our feet?