Wisconsin Rapids police say it was just a sonic boom – nothing to worry about. Military aircraft are conducting the 2021 Northern Lightning exercise out of Volk Field in Camp Douglas. The 11-day training event started August 9th and wraps up today (Friday). One of the 60 military aircraft involved startled people by breaking the sound barrier early Thursday morning. Military officials report there are six Air National Guard units, two Air Force units, two Navy units, and a Marine unit involved, totaling 1,600 service members.