Peering at the Greathouse Portal at the Black Diamond Mines Regional Preserve south of Antioch, California.  The area was rich in production of silica sand & coal (the Mount Diablo coal field being discovered in 1855).  During its heyday, the area had 12 mines that supported thousands of miners and their families from around the world in five town sites. Four million tons of sub-bituminous and lignite coal was extracted from the Mount Diablo coal fields until the quantity, price and quality was overtaken by a larger operation with higher-quality anthracite coal from Washington State. The townsites were abandoned and houses moved when the mines closed around 1906. 

In the 1920s, a mine producing high-quality silica sandstone was started by owner Marvin Greathouse on a hillside above the Somersville townsite. He sold the product to the Hazel-Atlas Glass Company which operated a plant in Oakland manufacturing glass containers. Hazel-Atlas eventually purchased the mine and operated it until about 1945. 

In this view is the Greathouse Portal, to which several hundred feet underground are today a museum.  The area is only opened from Spring to Fall during the weekends.

From: https://www.kqed.org/science/23430/bay-areas-coal-mining-history-at-the-black-diamond-mines-regional-preserve

https://en.wikipedia.org/wiki/Black_Diamond_Mines_Regional_Preserve