Clues: Presents another key to the way science works: reconstructing the past with the evidence of the present. To illustrate this, Morrison follows a geologist who uses a simple hammer to test a sample and infer the history of Amethyst Mountain in Yellowstone National Park. For the rest of the intriguing story, an elaborate computer-controlled drill ship is required to piece together the clues to a new and dramatic hypothesis: that five million years ago the Mediterranean Sea was an arid salt desert formed as the straits of Gibraltar closed into a huge natural plug that shut off the Atlantic waters.