Ghost Tunnel, Black Diamond Mines - Photo: Nick Fullerton (CC) From the late 1800s to the early 1990s, as many as 900 miners at a time labored to remove nearly 4 million tons of coal—“black diamonds”—from the ground in what is now Black Diamond Mines Regional Preserve near Antioch in Contra Costa County. As interesting for its human history and artifacts (mine shafts, cemeteries, crumbled buildings) as for its 65 miles of pet-friendly trails, this 5,375-acre open-space oasis, part of the East Bay Regional Park District, is an off-the-beaten-track find for DogTrekkers. The hilly, oak-studded landscape reminiscent of the Sierra foothills melds into oak chaparral and evergreen forest to keep things interesting for you and Daisy. Read more.