Fossils are remnants or traces of an animal or plant of a past geologic age that have been preserved in various ways in the Earth’s crust. From earliest times people must have seen fossils, but the first reports we have on the subject are from the ancient Greeks. Xenophanes of the early Ionian school is said to have noticed fossilised sea creatures high on mountains; he correctly interpreted this as meaning that these mountains had once been under water. Later, Herodotus reached the same conclusion regarding fossilized clam shells, but he misinterpreted other fossils. For example, he associated the fossilised bones of large creatures with mythical animals or with giant humans.