Same thing with diamonds, there's a lot of them hidden below but we're not advanced enough to extract them efficiently.
there are a lot of them above ground. diamonds arnt actually rare at all. that's a myth. the only reason they have such a high value is because the de beers diamond cartel has monopolized the supply and holds them off of the market inorder to artificially inflate the price.
No longer 100% correct, times have changed:
The Incredible Story Of How De Beers Created And Lost The Most Powerful Monopoly Ever
http://www.businessinsider.com/history-of-de-beers-2011-12?op=1...
But more recently, countries with enormous stockpiles of their own, like Russia, Canada and Australia, have refused to cooperate with the single channel system.
These problems, along with issues of flat prices, forced De Beers to switch up the company's strategy. In the last decade De Beers has moved away from rough-diamond supplying and controlling the entire industry, instead focusing on promoting its own brand of diamonds and retail stores.