Some deep-diving whales, like orcas, can change their internal ear pressure to match the pressure around them, which helps them hear much better. We made a hydrophone that could do the same thing. Its sensor has three silicon diaphragms. Each is one hundredth the width of a human hair and is covered with thousands of tiny holes that let water pass through. The deeper the sensor goes, the more water flows in, equalizing the pressure and making the sensor much more sensitive to sound. That solved one problem, but it also created a new one. The diaphragms move very little when sound hits them, especially when it's deep, because water is not very compressible. The quietest sounds in the ocean will move a diaphragm just 0.00001 nanometers; that's almost one 10,000th of an atom's diameter. The motion is so subtle, the approach seemed like a dead end. But it turns out it's possible to detect that movement with lasers.