Maybe you’ve heard of geocaching, the wayfinding sport that’s swept America since 2000. The basic game involves hiding and finding secret packages in public spaces, all of which — GPS coordinates of the hides and logging of the finds — is administered via the geocaching.com website. My first exposure to geocaching was during a 2004 outing to remote Idria, California, where my friend Jef made a stop to visit and log this ammo-box cache in a roadside boulder pile. All he knew was the latitude and longitude.
The geocache visitor may find a paper logbook inside along with various trinkets.
As with a Little Free Library, visitors may take a trinket and leave one for a future geocacher.
I did not take up this hobby until last month, when I decided to take part in the Geological Society of America’s EarthCache variant of geocaching. (I’ve been a GSA member since the 1980s.) An EarthCache is a nonintrusive, virtual geocache without a container or trinkets, just a set of coordinates attached to a little free geology lesson about that location. You find your way there, observe the scene with the help of the lesson, and log the cache by submitting the answers to a set of questions posed by the cache’s creator.
Naturally I wanted to set my EarthCaches in Oakland. My first two EarthCaches have been approved, and as of this writing the third is under review by the GSA team. [Late update — here’s the third.] The first is up in the high hills, and the second is near Montclair. If you’re a regular geocacher, go for it! (There are a half-dozen other Oakland EarthCaches besides my two.)
If you aren’t a geocacher yet, you’ll find when you follow those links that the locations are censored, but the descriptions are visible. That means you can play a reverse version of the game if you like: from the descriptions, figure out the location and post it in a comment.
If that sounds fun, here are hints for each EarthCache. These are photos that visitors have sent me to document their visit. This one is from the first EarthCache.
And this one is from the second.
For Galaxy Brain status, tell me the locations without visiting the EarthCache pages.
This is a side project that I’ve spun off the book project, which is coming along nicely.
Oakland Geology 
15 March 2022 at 11:09 am
It only took me 11 years to carry out this idea: https://oaklandgeology.com/2011/02/24/oakland-earthcaches/
14 March 2022 at 9:08 am
The East Bay Regional Park District has gone through various phases of paranoia over the presence of geocaches. Under the prior police chief, park staff were apparently instructed to report geocaches to the police. There was concern that the bomb squad would need to be summoned. Perhaps if the cache is in a transparent container with a prominent label “Geocache”, an overreaction could be avoided. Just sayin’. Not my sport! Have fun.
[EBRPD permits all kinds of geocache: https://www.ebparks.org/recreation/geocaching
EarthCaches require landowner permission. Until I hear back from the City of Oakland, I’m not putting any in city parks, although there are some prime features there. — Andrew]