In a pitch-black cave, bats can’t see much. But even with their eyes shut, they can navigate rocky topography  at incredible speeds. This is because a bat’s flight isn’t just guided by its eyes, but rather, by its ears. It may seem impossible to see with sound, but bats, naval officers, and doctors do it all the time, using the unique properties of ultrasound. All sound is created when molecules in the air, water, or any other medium vibrate in a pulsing wave. The distance between each peak determines the wave’s frequency, measured as cycles per second, or hertz. This means that over the same amount  of time, a high frequency wave will complete  more cycles than a low frequency one. This is especially true of ultrasound, which includes any sound wave  exceeding 20,000 cycles per second. Humans can't hear or produce sounds with such high frequencies, but our flying friend can. When it’s too dark to see, he emits  an ultrasound wave with tall peaks. Since the wave cycles are happening so quickly, wave after wave rapidly bounces off nearby surfaces. Each wave’s tall peak hits every nook and cranny, producing an echo that carries a lot of information. By sensing the nuances  in this chain of echoes, our bat can create an internal map  of its environment. This is how bats use sound to see, and the process inspired humans to try and do the same. In World War One, French scientists sent ultrasound beams into the ocean to detect nearby enemy submarines. This early form of SONAR was a huge success, in large part because sound waves  travel even faster through mediums with more tightly packed molecules,  like water. In the 1950s, medical professionals began to experiment with this technique as a non-invasive way to see inside a patient’s body. Today, ultrasound imaging is used to evaluate organ damage, measure tissue thickness, and detect  gallbladder stones, tumors, and blood clots. But to explore how this tool works in practice, let’s consider its most well-known use—  the fetal ultrasound. First, the skin is covered  with conductive gel. Since sound waves lose speed and clarity  when traveling through air, this gooey substance ensures  an airtight seal between the body and the wand  emitting ultrasound waves. Then the machine operator begins sending ultrasound beams into the body. The waves pass through liquids like urine, blood, and amniotic fluid without creating any echoes. But when a wave encounters a solid structure, it bounces back. This echo is rendered as a dot  on the imaging screen. Objects like bones reflect the most waves, appearing as tightly packed dots  forming bright white shapes. Less dense objects appear  in fainter shades of gray, slowly creating an image  of the fetus’s internal organs. To get a complete picture, waves need to reach different depths in the patient’s body, bypassing some tissues  while echoing off others. Since longer, low frequency waves  actually penetrate deeper than short, high frequency ones, multiple frequencies  are often used together and composited into a life-like image. The operator can then zoom in and focus on different areas. And since ultrasound machines send and  receive cascades of waves in real time, the machine can even visualize movement. The waves used for medical ultrasound range from 2 million to 10 million hertz— over a hundred times higher than human ears can hear. These incredibly high frequencies create detailed images that allow doctors to diagnose  the smallest developmental deviations in the brain, heart, spine, and more. Even outside of pre-natal care, medical ultrasound has huge advantages over similar technologies. Unlike radiation-based imaging  or invasive surgical procedures, ultrasound has no known negative  side effects when used properly. At very high levels,  the heat caused by ultrasound waves can damage sensitive tissues, but technicians typically use the lowest levels possible. And since modern ultrasound machines can be small and portable, doctors can use them in the field— allowing them to see clearly in any medical emergency.