During World War II, the south end was the second largest naval training ground in the world and the largest "city" in the state. Built as a result of the Japanese attack on Pearl Harbor, the training station is now Farragut State Park. The lake's Large Scale Vehicle Range is still used by the Navy's Acoustic Research Detachment for sonar testing with large-scale submarine prototypes on the Pend Oreille Calibration Station. The significant depth gives acoustic properties similar to the open ocean. Background noise is less than at ocean testing locations, and the sound signatures being investigated cannot be monitored by foreign governments in international waters.