A preface: don’t do this on a road bike, it was very dumb and definitely not a good idea.
If you do, take lots of water and some lights.

Route, from Santa Barbara: up the 192 to Gibraltar, up Gib almost to La Cumbre Peak, turn off onto the gravel/fire road right after Gib becomes Camino Cielo - this road onto which you’re turning is rather confusingly named Gibraltar Road; descend until you hit the forest route (FR 5N18); there will be a gate on the right side, go through it and around the lake along the forest route until you hit the mine. Gaze in awe, eat a sandwich, then turn around and do that whole thing in reverse.

The mine itself has been fenced off, but you can still get pretty close by walking further in toward the lake once you see the mine. The trail should be visible once you’re close enough, when I did this it required no bushwhacking. There’s a bench up on the hill overlooking the lake, and of course the mine structures (a main building plus some rail cars and other stuff nearby).

When I rode this, I did it on a road bike, and that was not optimal in terms of safety or comfort, but it was really fun and I like telling the story. About a half mile into the forest route (after the gate, once the trail turned from sandy rock to actual brambles and unrideable rock), I left my bike near a bush and picked it up on the way back. This was mostly out of discouragement from being almost out of water and hiking in road cleats through bushes, over fallen rock piles, and across streams. With the right footwear and about two more liters of water, I’d say it would be doable with the bike on your shoulders all the way to the mine, but is it worth it? My personal answer is no.

If you’re going to follow my lead, I suggest extra water and hiking shoes, in addition to your normal lights and extra tubes. Also, sunscreen, and more often than the tube says. A lot of that gravel descent and the lake trail is exposed, and climbing back out to Gibraltar takes a while. I saw a few people in cars, presumably park rangers or foresters, between Gib and the lake, but no one on the lake trail itself, so there genuinely isn’t a water source waiting for you along the way. I’ve heard rumors that there’s a small water box of some sort a bit of a ways away from the junction of the sandy-rock Gibraltar Road and the lake trail, but that story contained the words “lots of black widows” and even then I didn’t really see anything I would describe as probably full of water anywhere near that junction.

By the time I was descending Gib, it was already dark out, and since there’s no water, shelter, or cell reception for pretty much that entire climb, it’s all on you to make it out. Really, the entire thing is very much the definition of ‘you’re on your own,’ in all the good and bad senses of that phrase. Maybe bring a friend.

As with most of my long/adventure rides, I did this one unsupported with a small pack, and I couldn’t really see myself doing it safely with just pocket space.

Our team captain my freshman year of college coined (at least in our circle) the phrase “third-degree fun” for the concept of a ride that is horrible when you’re doing it but fun to look back on afterward. From this follow the concepts of “first-degree fun” (a ride that’s fun while you’re doing it) and “second-degree fun” (loose definition: kind of fun when you’re doing it, more fun immediately after). So this ride’s Patented R-Lo Degrees of Fun Rating System (TM) rating is: third-degree fun. Absolutely horrible by the end, but so much fun looking back.