On Nov. 12, someone moved almost 25,000 bitcoins, worth about $159 million at the time, to an online exchange. The news soon rippled through online forums, with bitcoin traders arguing about whether it meant the owner was about to sell the digital currency. From a report on Bloomberg: Holders of large amounts of bitcoin are often known as whales. And they're becoming a worry for investors. They can send prices plummeting by selling even a portion of their holdings. And those sales are more probable now that the cryptocurrency is up nearly twelvefold from the beginning of the year. About 40 percent of bitcoin is held by perhaps 1,000 users; at current prices, each may want to sell about half of his or her holdings, says Aaron Brown, former managing director and head of financial markets research at AQR Capital Management. What's more, the whales can coordinate their moves or preview them to a select few. Many of the large owners have known one another for years and stuck by bitcoin through the early days when it was derided, and they can potentially band together to tank or prop up the market.