One of the basic ideas behind Bitcoin is that it exists outside the control of any centralized power. GHash.IO, a particularly powerful organization of Bitcoin miners, turns out to have at times controlled more than half of all Bitcoin mining activity -- a development that some think poses a new threat to the system. The creation of new Bitcoins has increasingly become the preserve of sprawling groups known as mining pools. Just like miners looking for gold, Bitcoin verifiers don't know exactly where the rewards will be, but every once in a while, Bitcoin miners involved in verifying blocks of transactions will unlock a reward.