Mystery Boom in California

"A group of local scientists has uncovered some clues to the source of a mysterious disturbance that rattled San Diego County on the morning of April 4, shaking windows, doors and bookcases from the coast to the mountains.
"The scientists, based at Scripps Institution of Oceanography in La Jolla, say the disturbance was caused by a sound wave that started over the ocean and petered out over the Imperial County desert."
More here.