There is not much information about this cave on the internet, but the information I could dig up is extremely interesting. Apparently this was a cave first that later was used for mining. This area used to be inhabited by the Spaniards in the 1800's so it is quite possible that they used it for shelter. It was filled in with concrete by the city in the 80's but used to go extremely far in. I did find this off of the internet though
"Beginning in 1873, granite mines appeared in Mission Gorge. Light gray in color, the granodioritic rock extracted from these mines was used to construct roads, buildings, jetties, and dams. Blocks of granite from the Gorge were used in the construction of a breakwater in San Diego Bay and are still in place today. The family of Robert Waterman, Governor of California from 1887 to 1891, built the San Diego and Cuyamaca Eastern Railroad, which allowed the granite to be shipped economically from the mines.
Modern mining industries, which provide sand, gravel, and decomposed granite products, evolved out of the early granite mining business. Many firms extracted sand and gravel on lands now part of Mission Trails Regional Park, including J.B. Stringfellow, the Kenneth Golden Company, H.G. Fenton, and the V.R. Dennis Company.
In addition, the Morse Construction Company operated a dynamite magazine on park premises in the 1960s. Kumeyaay Lake and other nearby ponds are the by-products of these mining operations, created by the removal of rock materials in the late 1940s."
This area is filled with quartz, mica and black tourmaline. It is truly beautiful. The caves are nothing more than holes in rocks these days. They are interesting from the outside but there's not much to explore inside. The cave used to be quite extensive back in the day though, large enough to go water-tubing in, with stalagmites growing inside! The main mine we saw was filled with water so this is definitely possible! According to one of the local historians, there were two bodies that were found in this cave--one person got lost inside and the other murdered. This may have contributed to why it was filled in.
Personal Experience: Finally finding this spot was quite a feat for us! This was our second attempt looking for them. The first tip took us deep in a canyon with nothing but nature around us. A month later, after a much better tip, we headed out again and this time successfully found the mines! We had a very nice hike--although we went in early spring when the weather is mild. We passed a pond with a mini-waterfall. This area alone was pleasant enough to spend some time at. Would have been nice for a picnic. After a small hike up a hill we stumbled upon the mines! Both have been filled so there is not too much to see but there are crystals everywhere!