Loud and repeated night-time booms have bewildered San Geronimo Valley residents for the past month. The explosions, heard and even felt from Woodacre to Lagunitas, have prompted discussions on Nextdoor and multiple calls to the Sheriff’s Office, but so far remain unexplained. “[It’s] definitely a mystery that people in the valley are following closely because it’s so loud and perplexing,” valley resident Becky Bond said. Some neighbors have pinpointed the noises, which usually occur between the hours of 9 p.m. and midnight, as coming from near the former San Geronimo Golf Course or the Roy’s Redwoods Preserve. The Sheriff’s Office is investigating. “We haven’t been able to figure out who or what is causing it, but we think we have a general idea of where it’s coming from,” spokesman Brenton Schneider said. “We’re in the process of trying to catch the person who’s doing this.” He declined to say where deputies believed the booms were originating. In October, an explosion occurred at PG&E’s Woodacre substation when a mobile transformer fell off its trailer and caught fire during the storm. Yet most residents have reported the latest series of booms seems to be coming from the north side of Sir Francis Drake Boulevard, not near the substation.