San Francisco is 49 square miles, but contains more than 1,000 miles of sewer mains, running under every block. Take a trip with Bay Curious reporter Carly Severn into the depths of the city, and discover an otherworldly cave that has reached legendary status with some urban explorers. This story first aired in 2019.

Additional Reading:

• Exploring San Francisco's Hidden Tunnels (YouTube)
• Tunnels Under San Francisco? Inside the Dark, Dangerous World of the Sewers
• Read the transcript for this episode
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This story was reported by Carly Severn. The episode was also made by Olivia Allen-Price, Katrina Schwartz, Tamuna Chkareuli, Jessica Placzek and Rob Speight. Additional support from Cesar Saldana, Jen Chien, Katie Sprenger, Maha Sanad, Jasmine Garnett, Carly Severn, Joshua Ling, Holly Kernan and the whole KQED family.