Scientists are alarmed when they realise that a mass of trees in a Californian forest are suddenly dying off. Satellites that speed through space at 17,000 miles per hour have the potential to unveil all sorts of hidden wonders. Equipped with high-tech optics and radar, they reveal unseen worlds. Bizarre images can raise questions about the Garden of Eden, El Dorado, ancient cultures and hidden bases. Could satellites even capture the moment a plane broke apart mid-air? Experts unravel these enigmas and more on What on Earth?