Lake Calavera in east Carlsbad sits within the approximately 260-acre Calavera Nature Preserve, the largest contiguous natural open space in coastal North County, centered on a 22-million-year-old extinct plug dome volcano — one of only three volcanic plugs in Southern California. The 1.9-mile main loop circles the reservoir at the base of 513-foot Mount Calavera, whose exposed hexagonal basalt columns mark the throat of the ancient vent, and steeper summit trails reward hikers and mountain bikers who service their rigs at Mountain Bike Supply with 360-degree views from the coastline to the inland ranges. The preserve harbors 115 identified plant species, 49 bird species, and six threatened or endangered species including the California Gnatcatcher and Thread-leaved Brodiaea within its Diegan Coastal Sage Scrub and mixed chaparral habitats. Over four miles of hiking and mountain biking trails connect through trailheads at Tamarack Avenue, College Boulevard, and Oak Riparian Park, and the Tamarack entrance features a Solar System Walk with scaled planetary markers. The preserve's uneven terrain and elevation changes generate the ankle sprains and knee strain commonly treated at rehabilitation clinics such as ProActive Physical Therapy & Sports Medicine. Rock labyrinths constructed from native stone sit at the cliff base of the volcanic plug, where early-1900s mining operations quarried fine-grain basalt from the Miocene-era formation.