Approximately 90% of the world's cargo is transported by container - some 200 million cargo containers are transported between the world's seaports each year, constituting the most critical component of global trade. Since $12.5 trillion worth of merchandise is traded worldwide, the task of protecting cargo containers is critical. Given that some containers also transport dangerous biological, chemical and radioactive materials it is clear that container security is of tremendous importance. On March 14, 2004 ten Israelis were killed in the port of Ashod by terrorists hidden in a secret compartment in a cargo container, so it is clear that terrorists have begun thinking about using cargo containers as a vehicle for their attacks. We propose determining the acoustic fingerprint of a cargo container when it is first loaded, and then determining its acoustic fingerprint at various places as it moves to its destination. A significant difference in the fingerprint will cause the cargo container to receive greater attention, and give some indication of the changes that have taken place in the container.