There's a little-known pedestrian tunnel under Highway 13 providing access between Joaquin Miller Court and Monterey Boulevard. More significantly, it connects Dimond Canyon and the lower reaches of Joaquin Miller Park. It roughly follows the path of Palo Seco Creek which feeds into Sausal Creek.
In the bigger picture, it's part of a way to hike from the flats and lower hills of Oakland all the way to the Bay Ridge Trail.