On a Daly City beach just south of Fort Funston there's a large tunnel carved into the cliff. Bay Curious listener Francisco Alvarado noticed it one day while walking his chihuahua, Little Bean, down the beach. The tunnel is large enough for a person to stand up and several feet wide, so of course Francisco's mind started racing. What could this mysterious tunnel be? Is it a remnant of life long ago? Or could it be something as mundane as a drain outlet? We head to Phillip Burton Memorial Beach, as it's technically called, with a geologist to find some answers.

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This story was reported by Katrina Schwartz. Bay Curious is made by Katrina Schwartz, Gabriela Glueck and Christopher Beale. Additional support from Olivia Allen-Price, Jen Chien, Katie Sprenger, Maha Sanad, Alana Walker, Holly Kernan and everyone on Team KQED.