I first learned of the tunnel entrance in the Trade and Technology (T&T) Building when I began working for the Electronics Department in 1974. The T&T Building's tunnel crosses Terry Street (now named Martin Luther King, Jr. Way) to what was formerly East Hall (now, the Rendezvous Building) and runs down the hill toward the Administration Building.
In the winter, it's possible to see where some of the tunnels are, because the snow melts off the areas above the tunnels.
In 1986, an account of the tunnel system under "lower campus" (the area between 5th Avenue and 8th Avenue) was published in the student newspaper. The article is reproduced here, along with the accompanying map.
[Click on the images to enlarge them.]
The map does not show the tunnels that exist on "upper campus," the area east of 8th Avenue (now named Cesar Chavez Avenue).