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You’d never know it was there. Past a field dotted with hay bales, my driver turns up a small road that seems to go nowhere. When we reach a metal gate, a security guard comes out to greet me. Then he hands me a gas mask that looks like it should be in a war museum.

That’s not as crazy as it sounds.

In recent years, once-soaring prices of investment-grade vintages have slowed, but that hasn’t hurt Octavian: The wine is staying underground longer as clients wait for another surge. Photograph: Lauren Fleishman/Bloomberg Markets

I’m about to go 30 meters (100 feet) below rolling farmland outside Bath, England, to see more than £1 billion ($1.5 billion) worth of fine wine stored in an old mine that during World War II was a munitions dump. It’s been stashed there by clients ranging from Christie’s and Sotheby’s to financier Guy Hands.

At 93,000 square meters (1 million square feet)—think 22 football fields—Octavian’s Corsham Cellars is one of the largest storage facilities for fine wine in Europe. In the 1800s, it was an underground quarry that produced the honey-colored stone used to build the grand homes in nearby Bath. In the late 1930s, the U.K. Ministry of Defense took it over. Twenty-five years ago, it was turned into a wine vault, but technically it remains a mine, hence the gas mask requirement.

Aboveground, behind a security hut, a blue sign is emblazoned with Octavian’s logo: “Our world is fine wine.” From inside a nondescript green warehouse, a tiny industrial train transports the cosseted wine to the vaults, but the only way in for mere mortals is to walk 157 steps down a dimly lit mine shaft.

A map of the mine Photographer: ￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼Lauren Fleishman/Bloomberg Markets

As we descend, I’m hit by cool, humid air, a first hint of the care taken with the wine. In the vaults, the temperature is naturally about 13 degrees Celsius (55 degrees Fahrenheit), the ideal temperature for storing wine. Humidity is kept at 80 percent, plus or minus 5, and checked hourly. Filters cover light fixtures to reduce damage from ultraviolet radiation. There are motion detectors at the mine entrance; it would require an Ocean’s Eleven–type operation for thieves to haul anything out of here.

The wine is stored in ancient casks or by the case. Photographer: ￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼Lauren Fleishman/Bloomberg Markets

Wooden cases of awe-inspiring wine—a 1996 Château Latour here, a 2008 Château Lafite Rothschild there—are stacked high. Vincent O’Brien, the Irish-born Octavian managing director who’s my guide, says staff stood in amazement when a case of 1945 Château Lafite arrived earlier this year. Somewhere down here, dispersed among different chambers to reduce risk, are 1,000 cases or bottles dating from 1775 to 1800. “Are you getting the feeling we’re full?” O’Brien, 45, asks.

Almost. The number of cases stored by Octavian has swelled 35 percent during the past four years. In the wake of the financial crisis, wine investing became de rigueur as money rushed toward alternative assets. Standout Bordeaux vintages from 2009 and 2010 sent wine investors into a tizzy. Until 2011, 95 percent of the stored wine was from Bordeaux. Since the Bordeaux bull market ended in 2011, the Liv-ex Fine Wine 100 Index, which tracks prices for the 100 most sought-after wines, has fallen by about a third—and so has appetite for Bordeaux.

The vaults are decorated with World War II–era graffiti. Photographer: ￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼Lauren Fleishman/Bloomberg Markets

But that hasn’t hurt Octavian. Investors such as Hands are sitting on their collections. Hands recalls hauling a 1976 bottle of Salon Le Mesnil Champagne to Whistler, Canada, at Christmastime last year to celebrate the anniversary of meeting his wife at a New Year’s party. Having bought the bottle for £100 in 1999, he made the mistake of looking up the price now: £1,800. He ended up taking it back home. “I just thought, I can’t drink it!” he remembers. “It seems slightly sad in a way.”

The average time a case stays underground has increased to 8½ years, up from about six in 2011. Local residents shot down plans to build an extra warehouse directly above the mine to deal with increased demand. Instead, Octavian opened a high-tech spillover facility aboveground about 8 kilometers (5 miles) away in Colerne. At its two locations, Octavian houses £1.5 billion worth of wine, O’Brien says.

Octavian looks after wine worth more than £1 billion. Photographer: ￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼Lauren Fleishman/Bloomberg Markets

These days, clients are demanding photographs to document labels and the condition of wooden cases. A dent in the wood can lower values as much as 15 percent, O’Brien says. Octavian is in the process of building a new underground photo studio to churn out 6,000 images a month, up from about 500 in 2012. “We’ve got a different type of customer coming in the last couple of years,” he says. “They’re looking at it more from an investment angle.”

After an hour underground, I ask to be taken to the most expensive case in the vaults. “We’ll take you to the DRCs,” O’Brien says. No, he’s not talking about the Democratic Republic of Congo but Domaine de la Romanée-Conti, which produces some of the most expensive Burgundies in the world. “We’ve got a huge collection!” he says. After a half-hour search for a £120,000 case of 1985 DRC, I give up. I’m freezing, and I think I need a drink.