Back in the day, you could raceoption do bitcoin mining on your home PC. But as the price of bitcoins has shot up, the mining game has morphed into a bit of a space-race. Professional players, custom-designed hardware, and rapidly expanding processing power have all jumped on board.Today, all of the computers vying for those 25 bitcoins perform 5 quintillion mathematical calculations per second. To put it in perspective, that's about 150 times as many mathematical operations as the world's most powerful supercomputer.And mining can be pretty risky. Companies that build these custom machines typically charge you for the hardware upfront, and every day you wait for delivery is a day when it becomes harder to mine bitcoins. That reduces the amount of money you can earn.Why do these bitcoins have value? It's pretty simple. They've evolved into something that a lot of people want and they're in limited supply. Though the system continues to crank out bitcoins, this will stop when it reaches 21 million, which was designed to happen in about the year 2140.