It is not especially high, but because it stands alone above the surrounding plains, the view from the top is among the widest in the world, taking in the Sierra Nevada on a very clear day and, some say, more of the earth's surface than any peak apart from Kilimanjaro. The claim is a local legend, and nobody quite tests it.
The mountain has a long history. The Ohlone and Miwok peoples regarded it as sacred, a place of creation, and the Spanish gave it its name, which means devil, supposedly after a thicket where they lost a pursued group of indigenous people. The state park around it protects a rugged landscape of oak woodland, chaparral and bare ridges, with a rich show of wildflowers in spring and hills that turn from green to gold by June.
The road up the mountain, from Walnut Creek on the north side and Danville to the south, is a popular ride, with long curves, short steep sections and tight hairpins near the top, and it is shared with cyclists who love the climb. It is narrow and the surface is patchy, and there are drops at the edge. There is a small fee at the entrance, and the road can close in high fire danger or after storms. Watch out for deer and wild turkeys.
Alamo, a wealthy suburb, offers fuel, shops and a few cafés, and Moraga, with its Saint Mary's College campus and old adobe, has a pleasant village centre. The Lafayette Reservoir nearby has a path around it for a short walk. The towns have the full range of services, and the food is a mix of Californian cuisine and plenty of good Mexican places.
The ride to Mount Hamilton, further south, leads to the Lick Observatory, which has crowned the peak since the 1880s. From here, Mount Diablo works as a warm-up. Go early on a weekday to avoid the cyclists, and in the cool light of spring or autumn when the views are clearest. Summer afternoons can be hot, with a haze over the valley.