Prospectors say they have discovered a modern mother lode—not one buried deep within the earth, but sitting on the surface: the tailings of old mines, some dating back to California Gold Rush days. What these contain, though, isn't gold or silver. It's rare earth elements. Rare earth elements were discovered long after the Gold Rush ended, and are essential to the manufacture of a variety of high-tech products and weapons, such as MRI scanners, cell phones and lasers. "They're a group of elements that live at the bottom of the Periodic Table," Alan Koenig, a research geologist and with the U.S. Geological Survey, tells ABC News. "Typically, they co-exist together."