Outside the geologist community or among trail junkies, few know that San Diego was once home to an active volcano. These days it's a volcanic "plug," but 20 million years ago (before the Internet) lava flowed from Mt. Calavera in the northeastern part of Carlsbad.
A volcanic plug is created when lava hardens within an active volcano, and over time, completely blocks off the flow of lava. Once the volcano was extinct and the surrounding softer rock wore away, the erosion resistant lava was left behind in a distinctive upstanding formation.
There are just three such formations in all of Southern California. The process that created the Mt. Calavera volcanic plug was part of larger geologic changes in California that resulted in formation of the San Andreas Fault.
Our ancient volcano is part of the Lake Calavera Preserve. As if an extinct volcano isn't cool enough, the preserve also includes more than four miles of hiking and bike trails, a lake created by a dam (which dates from the World War II era), multiple stone labyrinths and a cave.
The cave is actually an old exploratory mine, known as an "adit." It was mainly used as a rock quarry for valuable minerals. There are at least three labyrinths in the preserve, and I advise a look on Google Maps to get coordinates. And, if you visit and plan to hike, be sure to bring an adequate supply of water, as it gets hot in the summer and fall.
The preserve is home to nearly 200 plant, bird, mammal and amphibian/reptile species, including several identified as endangered. Notable among them are the Thread-leaved Brodiaea, the Orange-throated Whiptail and the California Gnatcatcher. The gnatcatcher is perhaps the most threatened of these species because 85 percent of its natural habitat, Diegan coastal sage scrub, has been destroyed by development.
Lake Calavera covers 400 acres and is managed by the Carlsbad Municipal Water District. Fishing is allowed in selected parts of the lake, but be advised you need a permit. Rumor has it that there are bass, bluegills and crappie in the lake.
To see more off-the-beaten-path places in San Diego, go to hiddensandiego.net.