Photograph by Mike Chapin

No spotter, just water.

Who needs a gym, or even a treadmill, when you’ve got … this?

Tucked between Carlsbad and Oceanside, Lake Calavera Preserve is a natural fitness center, with miles upon miles of hiking trails, a picturesque lake, and a volcanic plug that’s a great place for a leg workout — a half-mile climb that’s nearly straight up. The topography is remarkably varied. There’s a nearly two-mile hike around the lake, which is home to egrets, herons, ducks, and coots. Portions of the trail are shaded by trees. Further east, there are hilly red-sand trails straight out of Sedona. And on the northeast corner is the Oak Riparian Trail, a stunning walk along a little river that makes you think you’re somewhere in the backcountry, like Cuyamaca. Two huge granite outcrops, jutting out from beneath towering oak trees, are ideal spots for family or group photos.

Lake Calavera Preserve covers about 260 acres and is the largest contiguous natural area in coastal North County. This large plot of open space is bordered on the north, west and east by residential developments, while to the south lies an even bigger natural area, the Carlsbad Highland Ecological Reserve, which is owned by the state and administered by the California Department of Fish and Wildlife.

The preserve is part of Carlsbad’s Habitat Management Plan – a regional conservation plan used to comply with federal and state laws to protect threatened and endangered species. It’s the largest of 13 city-owned nature preserves in Carlsbad and home to four sensitive plant species and 12 sensitive animal species. In addition, there are 17 native vegetation communities on site, including Diegan coastal sage scrub, southern mixed chaparral, southern willow scrub, mulefat scrub and freshwater marsh.

Now, about that volcanic plug. What locals call Mount Calavera is what’s left of an ancient volcano that erupted sometime between 13 million and 18 million years ago. A cut in the volcano dates back to when rock was mined to construct Calavera Lake Dam during the World War II years, turning the lake into a reservoir. Mira Costa College students often venture to the volcano on field trips; according to a college website, “the magma that created the volcano forced its way to the surface along a fault where it erupted, creating a cinder cone, with lava flows. The surface the volcano formed on was a coastal plain blanketed with flat-lying sedimentary rock that was originally deposited in or along the shores of a shallow marine embayment that extended inland to the base of the mountain ranges to the east.”

Hiking to the top of the plug affords visitors panoramic views all the way to the Pacific Ocean. There are three labyrinths, the origins of which are unknown.