Adventure to the Legendary Crystal Cave of San Bruno Mountain

South San Francisco, CA      May 21, 2014    By Andy Howse, Founder of Open the SF Watershed

Overlooking the area Photo: ROGER CAIN

Overlooking the area

Imagining the landscape that is the San Francisco Peninsula looked in bygone times is one of my favorite pastimes.  Seeing the world from this perspective turns everyday places into historical gems that provide a view of a former landscape. They are the living clues that connect to the stories of the areas that surround them that have changed.  Some are well documented, others are not, and are in need of some educated guesses. One of these latter type places is a local legend, The Crystal Cave of San Bruno Mountain.


We adventured to this hidden piece of history, on assignment of sort, for Everything South City. We were very fortunate to have an experienced miner, Roger Cain,  in our company of three when we went to explore the Crystal Cave of San Bruno Mountain.   I have been to the cave several times, but this was the first time that we had a person of such knowledge involved, and the first time we agreed to document our little expedition.

Making their way by rope Photo: ROGER CAIN

Making their way by rope

Making their way through Stinging Nettle and Poison Oak Photo: Andy Howse

Making their way through Stinging Nettle and Poison Oak
Photo: Andy Howse













“These tailings indicate that this was definitely man made, probably done by hand (pick axe). Although this wasn’t a large operation, there are probably records of this somewhere” Roger, our prospecting hobbyist said staring at the piles of quartz crystal infused rocks that lied outside the cave and then down the cliff to the creek below. “At some point they would have had to cut a trail clear up this valley, large enough to bring animals and equipment up” he added. Roger was not positive that they were looking for gold, but that that was his theory. He noted they may have been looking for other minerals, but he doesn’t believe they found anything, as the mine was abandoned, leaving behind what is now known as the Crystal Cave. Also, had they found gold, they would have had to crush the ore on site or carry it out to a place where they could separate the gold from the rock. The cave roughly measured is around 30 feet deep and about six feet tall and hard rock all the way.


Roger explained that when miners are looking for gold they often look for quartz, that gold and quartz are synonymous.  When they found a gold vein and the ore was crushed, the gold would be extracted either by panning the gold or by leeching it out from the rock using quicksilver (mercury). To this day the creeks in San Jose are polluted with mercury and there are relics of the quicksilver mines in the hills, remnants of the prospecting economy that dominated our region a little over a century ago. The Crystal Cave tells stories of a time when mining speculators were looking for gold on San Bruno Mountain.

The cave measures 30' deep & about 6; tall.  Photo: ROGER CAIN

The cave measures 30′ deep & about 6; tall.

Crystals have been abundant on the mountain Photo: Andy Howse

Crystals have been abundant on the mountain
Photo: Andy Howse










Stinging Nettles keep explorers out Photo: ROGER CAIN

Stinging Nettles keep explorers out
Photo: Andy Howse















All but the trailhead to the Crystal Cave was grown over and although there was evidence of recent past human habitation (a weathered sleeping bag) it was clear that no one had ventured to the cave in months.


The land is alive 'Millipede' Photo: ROGER CAIN

The land is alive

The trail to the cave crosses a spring led creek fed by the underground aquifer beneath San Bruno Mountain. This small waterway cut The Devil’s Arroyo, the name of the valley where the cave resides. What was left of the trail was grown over by many local plants. A large portion of these plants are poison oak and stinging nettles. This proved to be a challenge for us, as was the slippery steep climbs. The trail follows the creek and then shoots up the hillside, leaving the creek behind.  It was fortunate for us that adventurers in years past had installed a network of ropes to traverse the mud and rock hill, and although we brought along ropes of our own, we did not need to use them. Of all the times I have been to the Crystal Cave due to the thick overgrowth of nettles and poison oak, this was by far the most treacherous.


We made it to the cave relatively unscathed and feeling a little more alive than when we started. As we stared down The Devils Arroyo into civilization less than a mile beyond, we felt the ironic feeling that although we were standing in what felt like wilderness, civilization was in very close proximity.


After Roger explained the likely history of this hole in San Bruno Mountain, I pointed at the trail that continued.  It is very grown over now, but I explained the trail eventually meets up with the creek again. Then the creek comes to a rock wall that is roughly fifteen feet high, and covered in moss as the creek disperses itself down its face. Above this wall is a small cliff where the creek comes out of the hillside. Resting, and staring at the nettle infested trail, we decided to not make the final leg of the journey on this day. Instead we took a roundabout trail to the top of the cave, and it was clear from this vantage point that the line of orange quartz runs vertically down the hillside. We rested and admired the quartz crystals and the wildflowers. Then we started the journey back to the urban environment we came from.

andy & Stasai ROGER

Andy & Stasai in front of the Crystal Cave
Photo: Roger Cain

into the cave

Ducking to enter the 6′ entrance
Photo: Andy Howse














The three of us who made this trek consider ourselves environmentalists, the type who “leave only footprints, take only pictures” so we did seriously discuss not writing about our little adventure. Ultimately we decided that the article should be written. To those that may be upset about the confirmation of this local legend that we would like to note that our photos purposely left out any

Crystal Cave

Crystal rocks left behind

landmarks that that could be used to triangulate its location. Also, the trail being as overgrown and filled with Poison Oak and stinging Nettles as it was, we thought it unlikely that anyone would make such a journey simply to fulfill bad intentions.


Local legends and history are important. They connect us to the places we live and connect us together as the history of this place we live belongs to all of us. That is why history is one of the broader themes of the Open the SF Watershed movement, so we can explore and tell the tale to our children the colorful history of the San Francisco Peninsula.  What you see, and what is beneath your feet has a few stories to tell, once you know them, they are yours forever.


-Andy Howse, Editor  Open the SF Watershed

Roger Cain contributed to this article

To learn more about the campaign to open the San Francisco Watershed to responsible public access please CLICK HERE  Friend them on FACEBOOK


If anyone has any documents to the history of these mines/caves, we’d be interested to hear about them so we can continue this story. Please share here or contact



Because we have heard many say they want to climb the mountain and explore and search for the cave this added information is important. Please take heed.

We would like to add a warning to this story for those who may go out searching on the mountain.


*To be successful you must be in good health and physically fit


*Do not go alone, always bring another person with you


*Let others know your plans before you head out


*Dress covered head to toe to protect yourself from Stinging Nettles and Poison Oak.


*Bring water & emergency supplies including rope, gloves, etc


*Carry in, carry out. Leave nothing behind but your footprints. Always respect the Mountain



20 comments for “Adventure to the Legendary Crystal Cave of San Bruno Mountain

  1. Editor
    May 25, 2014 at 4:29 pm

    This reply was made to Roger Cain who then shared it with us

    Christopher David English: Roger, I agree with your assessment on gold operation. The exposed quartz vein gives it away. I also did not see any horizontal drill marks on the walls telling me it probably was done by hand–as you also noted. I will throw one more twist into it… I think it was dug by Chinese imigrants returning from the railroads or the gold fields in the east. That is my theory.

  2. Editor
    May 27, 2014 at 4:37 pm

    We would like to add a warning to this story for those who may go out searching on the mountain.

    *To be successful you must be in good health and physically fit

    *Do not go alone, always bring another person with you

    *Let others know your plans before you head out

    *Dress covered head to toe to protect yourself from Stinging Nettles and Poison Oak.

    *Bring water & emergency supplies including rope, gloves, etc

    *Carry in, carry out. Leave nothing behind but your footprints. Always respect the Mountain

  3. Chris
    May 27, 2014 at 9:06 pm

    If you are allergic to poison oak stay away! There is a forest of poison oak on the hike and when I went to the cave in the 1975 I got the worst case of poison oak – covered from head to toe – my eyes were even sealed closed for a few days! It is a cool cave but not worth the gamble.

  4. June 3, 2014 at 1:35 pm

    Much thanks to Roger Cain and ESC for helping to put this together.

    If you do attempt this trip, please take care of yourself, and San Bruno Mountain as well.

  5. Dante
    June 20, 2014 at 3:47 pm

    The ooooold I went here in 1975!!

    July 29, 2014 at 11:59 am


    • Kristina Anderson
      September 6, 2014 at 12:46 am

      Hey Ed! Me and my fam wanted to go before weather gets foggy n wet…soonish like Indian Summertime…between Early Sept n Oct. The problem is I’m not REALLY sure I remember the way. I could pay u whatev for the guidance n I make a SUPREME picnic lunch (if I do say so myself!! 🙂 ===

    • Mike
      September 17, 2014 at 6:31 pm

      Peter Kirby’s relative?

    • ron Moon
      November 29, 2014 at 2:18 pm

      Is this Ed from oceana high school 1980/81 ? I live a third of the way up westside of mountain next to the retired landfill. Ron Moon

    • Ann Marie
      February 26, 2017 at 10:35 pm

      HI Ed,

      My friends and I would love to hike up to the cave. If you would be willing to give us anyour help on how to get up there that would be greatly appreciated!

      Thank you

    • March 5, 2019 at 2:29 am

      OMG I would love to have you guide me & my best friend to this cave. Today we made the trip 9 hours later half dead and dirty full of stickers wet numb feet… We unfortunately did not find the cave!!!! WE NEED YOU LOL

  7. Daniel Tuoto
    February 8, 2015 at 12:41 pm

    I ventured to this cave as a small boy living in Brisbane circa 1957-1958 with two of my school buddies.

  8. Mick Wedley
    May 30, 2015 at 2:53 pm

    A year later, but, earlier today me and a few friends decided to hike out and look for this very cool spot, after seeing this article. An hour later we were there a tough but very enjoyable trail. We took clippers with us and there were needed.

  9. Dante
    January 17, 2016 at 6:31 am

    The ooooooold I was there in the 50’s as a schoolboy.

  10. July 12, 2017 at 6:36 pm

    I grew up on the top of Schwerin Street in the Bayshore area now annexed to Daly City.I went up to Crystal Cave at least once a year beginning about 1963. We would hike up the hill and over the hill to the dirt road where we would meet at a huge fallen tree. From the fallen tree it was only a couple miles down into the valley and up the relatively hidden trail to Crystal Cave. I caught poison oak
    just about every time and had it so many times I began to get immune to it. We usually went up
    in the summertime, but a couple of times we went in winter .We avoided going in late August or
    September because the poison oak was bright red and angry and easily contagious. I learned later
    to take a shower immediately after the hikes to scrub the poison oak resin from my skin. There was a definite trail to the cave although there were bushes that had to be hacked down by our big sticks that we pretended were machetes . After spring time rains there was a nice waterfall just above the cave which ran into a creek. The creek ran down in the valley where Valley Drive is now, to a pond/lake. I found Native American artifacts such as arrowheads and eating utensils such as plates or bowls. There were quite a lot of Hawks and owls flying above in those days but we only saw one bobcat and mountain lion tracks only once.

    • Editor
      July 13, 2017 at 10:08 am

      Hi Jim,
      Thank you for sharing such a great story of your youth. Much appreciated.

  11. Ed Coffey
    December 14, 2017 at 8:14 am

    My boy scout leader took our troup to this cave in the 1980’s. He was a fireman and had found it a decade or two earlier while fighting a fire on the mountain. Great to see this article.

  12. Carlos
    May 21, 2018 at 3:46 pm

    If anyone knows the way to this cave I’d love to see it. I’m familiar with the challenges of this sort of terrain. In return I know some other very secret and equally interesting spots on the peninsula I could share.

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