Behind the guards, the blast doors and down corridors of reinforced concrete, sit the encrypted computer servers — connected to nothing — that hold keys to a vast digital fortune.

Argentine entrepreneur Wences Casares has spent the past several years persuading Silicon Valley millionaires and billionaires that Bitcoin is the global currency of the future, that they need to buy some, and that he’s the man to safeguard it. His startup, Xapo, has built a network of underground vaults on five continents, including one in a decommissioned Swiss military bunker.

Wences Casares

Photographer: Daniel Acker/Bloomberg

In the rarefied world of wealth management, Xapo is known for a client list studded with family offices, and for occasionally letting a journalist peekinto a stronghold to write about its security. But one secret has proven elusive: how much digital cash does it really hold?

Two Xapo clients said it houses roughly $10 billion of Bitcoin. Another person close to the venture called the figure an accurate approximation. Bitcoin’s price, after all, is hardly steady.

‘Patient Zero’

Even in the colorful world of crypto the cache is remarkable — amounting to about 7 percent of the global Bitcoin supply. It would mean Xapo, just 4 years old, has more “deposits” than 98 percent of the roughly 5,670 banks in the U.S. But as a custodian it’s regulated differently. The Swiss subsidiary is overseen by the self-regulating Financial Services Standards Association, which audits members to ensure they comply with anti-money-laundering rules. Xapo serves U.S. customers through a Delaware corporation that’s registered with the U.S. Treasury Department’s Financial Crimes Enforcement Network and is licensed in several states.

The outsize holdings underscore the faith that Casares — a serial entrepreneur nicknamed “Patient Zero” for stirring Silicon Valley’s interest in Bitcoin — has garnered among his acolytes and at major crypto investment firms, such as Grayscale and CoinShares.

Xapo has a network of underground vaults on five continents.
Photographer: Fabrice Coffrini/AFP via Getty Images

“Everyone who isn’t keeping keys themselves is keeping them with Xapo,” said Ryan Radloff of CoinShares, which has more than $500 million of Bitcoin stored at Xapo. “You couldn’t pay me to keep it with a bank.”

Xapo’s vault in Switzerland

Source: Xapo

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Xapo’s billionaire backers include LinkedIn Corp. co-founder Reid Hoffman and former Wall Street trader Mike Novogratz, who’s in the process of setting up his own cryptocurrency merchant bank. Their bet is that Bitcoin is here to stay, and so is its biggest scourge, theft.

The first rule of owning Bitcoin is to securely keep your private key — the code that lets you spend your coins. If thieves get it, they can loot your holdings in an instant, with no hope of recovery. Putting keys on a device connected to the Internet is both convenient and perilous: Hackers have proven adept at obtaining them from afar.

The most popular alternative is called cold storage, keeping the key in an offline device such as a thumb drive. But risks remain: Hackers have also proven adept at setting traps on computers to access cold-storage devices the moment they’re online. More traditional criminals have committed home invasions and kidnappings. Some Bitcoin tycoons have resorted to hiding their identities, fortifying their homes and studying self-defense.

Trading Desk

Xapo’s solution is to bury a cold-storage device in a mountainside and layer on electronic safeguards.

“They’re the first folks who recognized custodial and security functions would be key,” said Hoffman, whose venture capital firm Greylock Partnersled a $20 million investment in Xapo in 2014, a couple of years after Casares persuaded him to buy his first Bitcoin. “He made the pitch in the morning and in the afternoon I called him with an offer.”

At Xapo, retrieving Bitcoin from the vault takes about two days. The company verifies a client’s identity and authenticates the request before manually signing the transactions with private keys from multiple vault locations. Approval from three separate vaults is required for any transactions to be authorized. The company also offers customers a trading desk to buy and sell Bitcoin and created the first Bitcoin debit card to spend it.

Casares, 44, declined to comment for this story. After a flurry of publicity at Xapo’s launch, he has drawn big audiences at crypto conferences but largely shunned media interviews.

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Yet his persuasiveness is legendary. His evangelizing of Bitcoin is so pervasive in Silicon Valley that when Hoffman asked his family office to buy some, his banker asked when he’d spoken to Casares. Xapo’s advisers now include former U.S. Treasury Secretary Larry Summers, ex-Citigroup Inc. Chief Executive Officer John Reed and Visa International founder Dee Hock.