Think of them as the metal detectors of the digital age. Instead of combing beaches for gold, these experts use everything from supercomputers to hypnotherapy to reunite people with their lost cryptocurrency.

According to Chainalysis, a blockchain-analysis firm based in New York City, about a fifth of all bitcoin—around $20 billion—is lost, most of it permanently. When people purchase bitcoin, the details of their transactions are stored in software called a “wallet,” which they can unlock with a personal-identification number. Unlike an ATM PIN, this password can’t be recovered easily, since there is no bank to retrieve it.

In the early 2010s, when bitcoin was trading under $10, some investors were careless with their PINs. Now that the currency is trading around $6,000, a waylaid slip of paper, misplaced USB drive or damaged hard drive containing a password could be the key to a small fortune. (Bitcoin makes up the majority of missing cryptocurrency because it existed before the recent explosion in value, Chainalysis said.)

To the rescue: A small group of crypto hunters. Here, what they offer, how successful they are and what they cost.