A hundred feet beneath the streets of Manhattan's financial district, a 2 1/2-acre chamber dug from bedrock is being turned into a modern treasure vault.

The man-made cavern at the roots of the World Trade Center, part of an old railroad, has a guard station with machine-gun-proof glass, Halon gas for extinguishing fires and 14 closed-circuit-TV cameras for surveillance.

DataPort would be a safe place for diamonds, rubies or gold bullion. Instead, it will contain nothing but thousands of reels of magnetic tape -- in effect, duplicates of the memories of modern corporations.

DataPort and other storehouses across the country cater to their clients' growing nervousness about their dependence on computers that are vulnerable to fire, flood, sabotage, theft or simple human error.

Experts say a total computer wipeout with no backup plan in place -- an unlikely event -- might permanently cripple a business such as a bank or brokerage firm.

"A computer failure is probably the single most tragic business event that could happen to a company. You can't move product, you can't collect money, you can't ship, you can't collect premiums. You just can't function," said John Ratliff, vice president of marketing for Sungard Services Co. of Wayne, Pa.

In spite of the danger, no more than 1,000 of the roughly 14,000 data centers in the United States and Canada that use International Business Machines Corp. mainframes of the 4300 series or bigger have disaster plans that include off-site backup computers, estimates Ray Hipp, president of Comdisco Data Recovery Services Inc. of Rosemont, Ill.

Executives are reluctant to divert large sums of money away from pressing needs to guard against a disaster that may never happen, say Comdisco and Sungard, which are No. 1 and No. 2, respectively, in the business of supplying backup computers.

But auditors are stepping up the security trend by demanding before they give a clean bill of financial health to companies that the businesses have workable disaster plans, tested regularly and discussed at least once a year by the board of directors. The Internal Revenue Service has furthered the trend, imposing several rules governing secure storage of long-term records.

Stomachs churned in November when a software problem briefly prevented the Bank of New York, the nation's 18th-largest bank, from processing purchases and sales of government securities.

Trouble piled up so fast that the bank had to borrow an unprecedented $24 billion from the Federal Reserve Bank overnight to balance its books, pledging all its assets as collateral.

The glitch, though repaired by the next day, distorted statistics on the entire nation's money supply that week and prompted a congressional investigation.

"You used to be able to pull out the green eyeshades and yellow pads and pencils and somehow get back to the way you were. Those days are gone," said Comdisco's Hipp.

Companies such as New York's DataPort store backup copies of a client firm's computer records. The client can retrieve the magnetic tapes on short notice and load them into its computers to replace ones that have been damaged.

Unfortunately, a disaster that destroys data often destroys machines as well. Comdisco, Sungard and other companies charge clients a big fee for the right to use spare computers that they keep ready and waiting at "hot sites."

For those who cannot afford a hot site there is the "cold site," a room equipped with chilled water, electricity and phone lines that is ready to have a computer installed in it in an emergency.

Money is definitely an object in deciding the level of service required. Comdisco charges its biggest clients up to $20,000 a month for access to a big IBM mainframe and a variety of peripheral equipment, Hipp said.

Computer makers such as IBM and Digital Equipment Corp. also offer rescue services such as rush shipment of replacement computers and consulting by their in-house disaster experts.

The New York Stock Exchange relies on 13 computers to support minute-to-minute floor trading. Three could fail in a way that would bring trading to a halt, said Don Dueweke, the exchange's senior vice president of market operations.

"Our operation is so automation-dependent it becomes, to some degree, scary, and we're not different from a lot of other corporations," he said.

Sometimes a small failure serves to remind companies of how serious a big failure would be. At the New York Stock Exchange, for example, a snafu occasionally will cause trading to halt for 15 minutes or half an hour.

All the exchange's computers now are in one secure section of a building just off Wall Street, but Dueweke said it plans to split the machines between two sites to lessen the chance of an incapacitating disaster.

Like other big New York computer users, the stock exchange pays a fee to a center in New Jersey that maintains backup computers it can use on a moment's notice, and it stores backup records in a commercial storehouse in upstate New York.

An intruder with a screwdriver, possibly a disgruntled former employee, dismantled a data storage unit at Paychex Inc.'s office in Paramus, N.J., on New Year's weekend in 1983, destroying end-of-the-year payroll and tax records that had to be laboriously reentered by hand. A spokesman at company headquarters in Rochester, N.Y., while downplaying the sabotage, said Paychex might have lost some payroll customers as a result.

In recent years, good backup arrangements have prevented accidents from turning into computer disasters at Mazda Motors Corp. of America (a roof collapse in Compton, Calif.), Del Monte Corp. (a transformer fire at San Francisco headquarters that released PCBs), Northwestern National Bank of Minneapolis (a fire that damaged data center), and the U.S. Postal Service headquarters in Washington (a fire resulting in water damage).

The first data-storage centers, built in the 1950s and '60s out of fear of a Soviet nuclear attack, tended to be in mountain strongholds far from cities. Records were rotated at the leisurely pace of once a month.

The newer centers, like DataPort, tend to be more convenient to cities. Records are shipped in and out as often as several times a day, ensuring that a disaster would not punch a hole of more than a few hours in a company's data records.

Computer managers are getting headaches over another modern trend, the increasing decentralization of computing power, which makes it harder for them to ensure that all the computerized information in their company is properly backed up and secured.

Users of decentralized computers suffered worse than mainframe users when a fire at U.S. Postal Service headquarters in Washington in October 1984 resulted in widespread water damage from fire hoses and sprinklers, the service's Rick Weirich said.

Data-storage experts say the next trend will be for companies to send their backup records into storage electronically. Workers at the receiving end would record the transmissions onto magnetic tapes and place the tapes on racks.

In this case, though, high technology has its critics. It is hard to verify that records sent electronically are not coming in garbled, so companies run the risk of discovering at the worst possible time that their backup information is unusable, argues William Dreyer, a vice president of Iron Mountain Group Inc., a major data-storage company based in Boston.