Fossils are the remains of organisms which have endured for fantastic periods of time. Fossils can be bones or teeth or even plant or animal imprints preserved in rock since prehistoric times. The appearance of fossils in rock has been a source of wonder and fascination to man for centuries. The fossil of an ancient sea animal was even found among the possessions of a prehistoric man. Many people have tried to explain fossils. Aristotle believed they were the remains of living creatures, but thought the creatures grew in the rocks. Some people believed that fossils were placed in rocks by evil spirits. Other explanations were remarkably modern. For example, Herodotus, an ancient Greek historian, observed fossil seashells in the Libyan desert in 450 B.C. and guessed that the Mediterranean Sea had once reached much farther south than it does today.