The inverse scattering problem. A ship emits an acoustic pulse; it travels through the water, reflects off hidden objects, and returns as an echo. The round-trip time gives range (distance = ½ × speed × time), and echo amplitude hints at size. Locating and imaging the unknown objects from these echoes is the inverse problem — the same principle behind sonar, radar, ultrasound, and seismic imaging. Click in the scene to place objects, then emit pings and watch the reconstruction build from echo timing.