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.

VIDEO: Take A Trip Into San Francisco's Sewer System

Web story: Tunnels Under San Francisco? Inside the Dark, Dangerous World of the Sewers

Reported by Carly Severn. Bay Curious is made by Olivia Allen-Price, Jessica Placzek, Suzie Racho and Rob Speight. Additional support from Julie Caine, Paul Lancour, Patricia Yollin, Vinnee Tong and Ethan Lindsey.