Here at Bay Curious, we’ve received a lot of questions about tunnels under San Francisco.

Listeners have told us they’ve heard stories of secret passageways running under the city. They’ve asked us, what is the truth about them?

The first thing I should tell you is: They're absolutely real.

What Lies Beneath?

The myth of the underground — a silent world hidden under our feet — is an endlessly alluring one. There are, after all, very real labyrinths under major world cities. Like the infamous catacombs of Paris, lined with the bones of the city’s dead, or the terrifying catacombs under Odessa, in Ukraine.

Some people get so obsessed with the idea of tunnels that they go in search of underground adventure themselves. They call themselves “urban explorers.” If you hit Google looking for information on San Francisco’s particular underground, there’s a name that comes up again and again — an explorer named Sierra Hartman.

A photographer and writer, Hartman’s haunting photographs of shadowy spaces under S.F. are for many people their first clue that this particular world of tunnels really does exist.