When sonar first picked up a massive, stair‑like object on the floor of the Baltic Sea, the image looked less like geology and more like a movie prop. The structure’s circular outline, sharp edges and ...

Off the coast of Chile there’s an island filled with mysterious monolithic structures that hold secret powers. See more in this clip from Season 21, Episode 2, "The Top Ten Mysterious Monoliths." ...

One night about 60 years ago, physicist Enrico Fermi looked up into the sky and asked, "Where is everybody?" He was talking about aliens. Today, scientists know that there are millions, perhaps ...