Personal Experience: I am hoping someone can help out with the story about this spot. It is definitely a man-made cave and the exterior appears to be made out of concrete. This is NOT a safe cave to explore and the opening is extremely tiny. Still, a full-grown man CAN fit through it as we tested. We crawled only slightly inside to document what it looks like. This spot is probably coolest just to say you know where it is but should not be something you hang out inside. The hike is easy, maybe 15 minutes and has a pretty view.
Update: So I have received a little feedback on the cave. I am being told that over 30 years ago it was dug out by a homeless man. Teenagers and kids who had run away from home would sleep in it causing it to be sealed off by the city. At some point it was dug back out which is why the opening is so small!
After a bit of searching around we finally found it! Holy crap that's a tiny opening! If it didn't have "Dead Man's" written on it I possibly wouldn't have thought it was the right spot.
There are three inlets inside. As you can see, this could very easily collapse at any time: