About 40% 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.

According to 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

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.

“Bitcoin is a digital currency and not a security, so there’s no prohibition against a trade in which a group agrees to buy enough to push the price up and then cashes out in minutes,” said Gary Ross, a securities lawyer at Ross&Shulga.

Bloomberg writes that regulators “have been slow to catch up with cryptocurrency trading, so many of the rules are still murky.”

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