The authors explore various applications that can be enabled by transforming mobile devices into active sonar systems. Specifically, they show how to use microphones and speakers available on existing smartphones to monitor minute body movements such as breathing. Using this, they build systems that will demonstrate the feasibility of achieving contactless solutions for sleep apnea diagnosis. Building on their active sonar approach, they also demonstrate the feasibility of subcentimeter finger tracking for around-device interaction using existing smartphones with no additional hardware. The resulting finger tracking system breaks a fundamental tension between the shrinking sizes of wearable and mobile devices and our ability to interact with them and achieves this without the need for additional hardware.