Have you ever thought about what a dog smells as it stops to sniff at a tree? Or what a cat is watching as it stares intensely off into space? What about animals in the wild? What do they see, hear, smell, and feel? How do they perceive their surroundings? This is the illusive world. A world filled with fascinating stimuli that we are not equipped to detect. This is particularly true because we tend to rely so heavily on our eyes or ears. We are figuratively, and literally, blind to this part of the natural world. This part, which is full of stimuli we cannot perceive, encompasses the daily lives of so many animals. Beneath our feet are ants, moles, and spiders using vibrations to coordinate colonies and communicate danger. In the oceans, turtles, fish, and octopi are sensitive to magnetic and electric fields, as well as tasty morsels at the tips of their tentacles. In the skies, owls and raptors can see deep into a lake or pierce the night with highly sensitive eyes. This book brings