After a chat with Supervisor Sean Elsbernd, a few calls to the GGNRA, and an investigative bike ride later, I was no closer to the source of the sound. I talked to the guys at Aqua Surf Shop. I asked cops and park rangers. Someone thought the Office of Emergency Services was testing a tsunami alert. I speculated about the ghost of Robinson Jeffers, lurking out there in the dunes. Finally, I called the Coast Guard, and the night officer knew what I was talking about. "There's been a sand barge working just off the coast these past few weeks," he told me. "We've gotten a bunch of calls from people who think it's a ship that's run aground." Apparently, this sand barge had been busy dredging the shipping channel outside the Gate and dumping its load just off the beach near the zoo, and when the fog gets low, the barge lets off its own horn to alert passing vessels of its location. Mystery solved.
And so, as quickly as it came, the fog lifted. The traffic signs on the Great Highway creaked in the wind, the Doggie Diner dachshund presided mutely over Sloat Boulevard, and the summer days rolled on, quiet as the tide.