After some research, I have figured out how one might reach the Big C.
Step 1: Start at North Gate, the intersection of Hearst and Euclid
Step 2: Walk East along Hearst
Step 3: Keep walking along Hearst. The street turns into Cyclotron.
Step 4: Walk along Cyclotron and you soon see this sign. It should appear near a parking lot.
BTW, I find it pretty funny that this guy's face is censored on Google Maps:
Step 5: Keep walking a bit more and the fence on the right hand side of the road disappears. Investigate this site of disappearance.
Step 5, enhanced: Now you see the trail. Follow it.
The above discussion shows that North Gate and Big C are path connected, in theory. A practical verification of this fact is left to the reader. Probably this weekend.