The hook
A ten-million-year-old volcano lies in the Oakland hills — tipped on its side by the Hayward Fault so you can walk straight into its guts — and someone has laid stone labyrinths in the old quarry pits at the bottom.
01Why it's worth it
Sibley is the East Bay's strangest geology lesson made walkable. Round Top was an active volcano 10.2 million years ago; then the Hayward Fault's grinding folded and tilted the whole vent complex onto its side, so quarrying later sliced it open and a self-guided trail now leads you through the cross-section of a volcano's plumbing — lava layers, a volcanic neck, baked and reddened rock, all legible with the park's interpretive brochure. Down in the old quarry basins sits the second surprise: hand-laid stone labyrinths, the largest built in 1990 by a local resident as a "gift to the world," where hikers walk the spiral to meditate and leave small talismans at the center. Volcano and labyrinth together make it one of those quietly profound Bay Area spots that residents guard and visitors never find — thirty minutes from downtown Oakland, with big ridge views thrown in.
02How to experience it
Skyline Blvd at the top of the Oakland hills (small staging lot). Grab the interpretive brochure at the trailhead board and follow the numbered volcanic geology loop; drop into the quarry to walk the Mazzariello labyrinth. Combine with a Grizzly Peak sunset or the Tilden steam trains up the ridge.