Gold mining involves digging up tons of rock from the ground, using chemicals like cyanide, and then tossing all the rock somewhere. I took a scenic drive along CA-hwy 49 over the weekend, and saw some mining equipment. I also read about a mine that still has gold, but it's too expensive (and probably would raise lots of environmental concerns).
Someday I'd like all gold mines to use gold mining bacteria. They pull the gold out of the ore while leaving the ore in place. Let's dump tons of these bacteria down into the gold mines, let them do their work, and then pull up all the gold. Whee!
Update: [2006-07-14] Researchers have found gold-harvesting bacteria.