On the northern side of Anza Borrego there are the remains of an old calcite mine. In operation around WWII, they mined optical calcite for use in gunsights. It didn’t last long, since around the same time a synthetic alternate was discovered, making the mining of calcite financially no longer feasible. What is left are a series of trenches and pits that follow calcite veins through the hills.
The Calcite mine isn’t the only thing of interest along the old mining road that we hiked (the road is drivable, but you better have a well-equipped 4×4 and plenty of experience). On the return hike we explored a side slot canyon, one that some hiking books call the best slot canyon in Anza Borrego. It was a nice little diversion.