Vasco Caves Regional Preserve is so cool you can only get in via a guided tour. From the East Bay Regional Parks District Page:

Spectacular rock outcrops break suddenly from the rolling eastern foothills of Mount Diablo, providing magnificent scenery for hikers and geologists alike. Though ancient, the Preserve’s resources are extraordinarily fragile, and exist today because of the land’s isolation and past efforts to keep it secret.

The Preserve is home to endangered red-legged frogs, tiger salamanders, and fairy shrimp, and is habitat for kit foxes, eagles, and a variety of nesting raptors. Its robust grasslands are a successful demonstration of native bunchgrass restoration using sheep grazing. The archaeological sites of Vasco Caves Regional Preserve contain Indian rock art, part of a complex that reaches back nearly 10,000 years.

You gotta call 1-888-327-2757 (press option 2, then 3.) to reserve a place in a tour.

Alert reader Ann L. did the tour a few weeks back.

Here’s a Flickr slide show:

Dots on this Google map are for the two parks where tour shuttles set out from.

View Round Valley and in a larger map