Spearfish — The mystery of what caused Monday's loud boom remains. Some theories have been refuted, while more mysterious references have appeared. Shortly before 2 p.m., the boom was heard throughout the Black Hills. Some people said it shook their homes or businesses, rattling windows, and scaring them in several instances. But the noise was heard in a much larger area than the Black Hills. Responses to Tuesday's Black Hills Pioneer story reported hearing the noise from Western Nebraska to Southeast Montana. Kathy Griesse reported hearing the noise near the Agate Fossil Beds National Monument south of Harrison, Neb. She said it sounded like the noise came from the north and west of her. Additionally, she talked to people in Crawford, Neb., where people told her windows rattled at the sound of the boom; people in Whitney, Neb., also heard the noise. On the northern end of reports, Lane Pilster said he heard the boom at his ranch, 14 miles west of Alzada, Mont. This is about a 200-mile