City: Oakland, CA
Yes, there are volcanoes in Oakland, and it takes some effort to get on top of them. In fact, on the way there you may find yourself in "a maze of twisty little passages, all alike". Also in the area, there be tunnels. That should narrow it down.
If you've hiked this area, please share your story with the rest of us.
I will happily reveal the trail name and location on the date below if no one posts the right answer, and confirm same if anyone gets it right. I look forward to hearing from fellow urban explorers.