The Oat Hill Mine Trail follows the remains of a public road started in 1873 and and officially opened in 1893. It was blasted and carved out of steep mountainsides in the town of Calistoga. The road was used to haul cinnabar, which was discovered there in 1872. After World War II, the road was used by Jeep enthusiasts. After several deaths, the road was closed to vehicles in 1979. The trail is very rough with much rock. Great views with interesting geology and history. Wear boots. There is no water on the trail. I used 2 liters of water on a 78 degree day.
Calistoga is about an hour and 45 mile drive from Sacramento (Google Map). On Lake County Highway Highway 29, right after you pass Silverado Trail there is a small dirt parking lot where the trail head starts. A covered sign has the map and historical information. You can also pick up a paper copy of the map there also.
The highlight of the hike occurred when were were about a mile up. We spotted a large brown colored black bear lumbering onto the trail. He stopped for a moment, perhaps catching wind of us, before moving down the steep incline. He never made eye contact with us.