Xapo, a startup led by Argentine entrepreneur Wences Casares, is holding $10 billion worth of bitcoin (BTC) in a network of underground vaults around the world, Bloomberg reports.

Disconnected, encrypted computer servers holding the keys to bitcoin riches lie in guarded vaults with blast doors and reinforced concrete, according to the report.

Casares, known in Silicon Valley as “Patient Zero” for his role in stoking Silicon Valley’s interest in the digital currency over the past several years, has the trust of major crypto investment firms, such as Grayscale and CoinShares. His vaults — including one in a decommissioned Swiss military bunker — hold about 7 percent of the world’s bitcoin supply, according to the story.

Xapo has billionaire investors, including LinkedIn co-founder Reid Hoffman and former Wall Street trader Mike Novogratz, who is setting up his own cryptocurrency merchant bank, Bloomberg reports.

The valuable digital currency, most recently worth about $9,348, is highly susceptible to theft. Holders must keep their private key safe from hackers who can instantly empty their accounts without leaving a trace. Keeping the computers disconnected, called “cold storage,” keeps thieves from accessing the cryptocurrency while it is offline, Bloomberg reported.

“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,” he tells Bloomberg. Read more here.