Deep Dark is a tunnel under Downtown San Luis Obispo that the San Luis Obispo Creek runs through. It is sometimes used by the homeless as a place to sleep for the night, used by local Taggers as a canvas for Graffiti, and used by all as a place for adventure and shenanigans. For an example shenanigan see the Sewer Stomp page.  To get in, either enter by walking upstream in the creek from the mission plaza, or start behind black horse coffee in downtown SLO and walk downstream to the mission.  Check it out!  It's awesome.

Looking out into Deep Dark from the graffiti offshoot entrance.Awesome.The graffiti offshoot tunnel is on the right side of Deep Dark shortly after you enter from the East end.