The simplest explanation of what Borax is and how it's obtained I found on Discovery. Here's a short quote from them:
Most of the world's supply of borax comes from Death Valley in southern California. Borax is also taken from open-pit mines in the nearby Mojave Desert, where miners strip away the covering ground to expose the borax bed. Workers use explosives to blast loose the solid borax. The large chunks of borax are crushed and dissolved. This solution goes through many purification steps until borax crystals are obtained. Borax is also obtained from "dry" or "bitter" lakes. The brine, which contains many salts other than borax, is pumped from the lake into containers. The solution is allowed to stand in vats to separate the borax from the heavier salts, which sink to the bottom. The remaining brine crystallizes, and the borax is refined.
Offered by Sol.