A high-power acoustic sensor, capable of detecting and tracking persons through steel walls of cargo containers, trailer truck bodies, and train cars, has been developed and demonstrated. The sensor is based on a new concept for narrowband mechanical-impact acoustic transmitters and matched resonant receivers. The lightweight, compact, and low-cost transmitters produce high-power acoustic pulses at one or more discrete frequencies with little input power. The energy for each pulse is accumulated over long times at low powers, like a mousetrap, and therefore can be operated with ordinary batteries and no power conditioning. A breadboard impact-transmitter and matched-receiver system that detected human motion through thick walls with only rudimentary signal processing is described, and results are presented. A conceptual design is presented of an acoustic through-the-wall sensor, costing about $10,000 per unit and capable of remotely and non-intrusively scanning steel cargo containers for stowaways at a rate of two containers per minute. Advantages of acoustic through-the-wall sensors over radar are: Sound penetrates metal walls; and acoustic sensors are sensitive to small and slow motions, and so can detect stationary persons by breathing motion alone. Other attractive features include: high-resolution locating and tracking; portability; low cost; quick and easy preparation and deployment; and near-real-time data processing and display. These features provide a robust stand-alone through-the-wall surveillance capability or an excellent complement to a radar sensor.