The value of all the bitcoins sitting in underground bunkers owned by wealth-management startup Xapo has been the subject of intense speculation ever since the company first allowed journalists into the catacombs where the family offices of the world's wealthiest people stash their digital gold.

Now, Bloomberg – after conversations with two purported clients – has produced a figure: Somewhere around $10 billion. Several sources said that number would be “a good approximation” – though the price can still be volatile.

Interestingly enough, with its reputation unmatched by other firms, Xapo has in effect created something more than just a bitcoin vault. It's essentially the first private bank for rich bitcoiners.

Because in addition to its security offerings, Xapo also provides customers with a bitcoin debit card and access to a bitcoin trading desk.

Already, the company's holdings – which constitute roughly 7% of the global bitcoin supply – are higher than 98% of the 5,670 banks in the US.

“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.”

Founded by Argentine entrepreneur and PayPal board member Wences Casares, who is widely credited with turning the Valley's VC billionaires on to bitcoin, the company has amassed a network of underground vaults, including a decommissioned Swiss military bunker.

Thanks to Cesares' reputation, Xapo has managed to attract venture investments from some of the Valley's biggest names, including LinkedIn Corp. co-founder Reed Hoffman and former Wall Street trader Mike Novogratz, who is, of course, is in the process of setting up his own crypto “merchant bank” that might end up competing with Xapo.

It also has relationships with major crypto investment firms, such as Grayscale and CoinShares.