For more than a century, people in the Bay Area — and especially the East Bay — have puzzled over the existence of stone walls scattered on ridges from near San Jose north through the Berkeley Hills. Sometimes the walls are built in long straight lines. Sometimes they form angles. Occasionally you’ll find rectangular or circular constructions. "Who built these things? How long ago? And why?" asked listener Eric Haven. It's a tougher question to answer than you might imagine, but reporter Dan Brekke does his best.

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This story was reported by Dan Brekke. Bay Curious is made by Olivia Allen-Price, Katrina Schwartz, Amanda Font and Brendan Willard. Additional support from Paul Lancour, Christopher Beale, Cesar Saldana, Jen Chien, Jasmine Garnett, Carly Severn, Jenny Pritchett and Holly Kernan.