A while back I thought I saw somebody looking for information about a Western Pacific tunnel in San Francisco. Might not have been here... But anyway, the tunnel was through Potrero Hill, and collapsed in 1960 after a fire. Recently found this interesting but very brief web page that mentions it complete with a vintage photo of the collapse pit.
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foundsf.org]
Enjoy! The main web site turns out to have a lot of interesting tidbits about SF history and neighborhoods, including some streetcar pix that I haven't seen elsewhere.