Every year the International Union for Conservation of Nature (IUCN) updates its list of threatened species and, for someone not familiar with the subject, this list is an important wake-up call. Once a species disappears from the face of the Earth, there is no going back. The top spots on this list include common favorites, such as gorillas, tigers, polar bears, and tuna, but also covers lesser known but equally astonishing species like the saola, the gharial, and the smoky flying squirrel. This guide explains which animals will probably disappear next and why it may be too late for others.