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Bitcoin's price soared past $56,000 on Friday, pushing the combined value of all bitcoins past the $1 trillion mark—at least in theory. That's more than the market capitalization of Facebook, though Apple, Alphabet, Amazon, and Microsoft are worth more. Bitcoin's value has almost doubled from $29,000 since the start of the year.

I say "in theory" because not all bitcoins are actually available for use. Each bitcoin (or fraction of a bitcoin) is secured by one or more cryptographic signatures. To transfer a bitcoin, you need to know its corresponding private key. And some of those keys have been lost—and it's impossible to know how many.

For example, a British man says he threw away a hard drive in 2013 that contained the keys to 7,500 bitcoins—worth around $400 million at today's price. He sought permission to excavate the dump containing the hard drive, but the request was denied.