Just east of the Caldecott Tunnel, a park on Skyline Boulevard lets you walk through the remains of a volcano.
The volcano
Round Top, at 1,761 feet, is an extinct volcano in the Oakland Hills that began erupting 10.2 million years ago, according to Wikipedia. The preserve is home to at least two stone labyrinths, both of recent origin. UC Berkeley professor emeritus Garniss Curtis studied its exposed volcanic rocks. The park district offers a self-guided volcano tour.
History
The district says Sibley, originally called Round Top Park, is one of its original parks along with Temescal and Tilden, dating to 1936. It was later named for Robert Sibley, who helped found the district and served 10 years on its board. Wikipedia says an expansion brought it to about 1,318 acres.
Visiting
The main staging area is at 6800 Skyline Blvd. in Oakland, and there are entrances on Old Tunnel Road and Pinehurst Road. Parking is free. Hours are 7 a.m. to 10 p.m. from March to October and 7 a.m. to 6 p.m. from November to February. Round Top Road is paved to the summit, and there is a 600-foot paved wheelchair path.