On the outskirts of the northern California town of Grass Valley, a massive concrete silo looms over the weeds and crumbling pavement. Nearby, unseen, a mine shaft drops 3,400 feet into the earth. These are the remains of Grass Valley’s Idaho-Maryland Mine, a relic from the town’s gold mining past. Numerous mines like this one once fueled Grass Valley’s economy, and today, Gold Rush artifacts are part of the town’s character: A stamp mill, once used to break up gold-bearing rock, now guards an intersection on Main Street, and old ore carts and other rusty remnants can be spotted in parking lots and storefronts around town.