Under the rolling hills and pastures of western Pennsylvania lies a treasure trove for the 21st century. Where once miners used dynamite to blow out chunks of limestone for use in the steel industry, data centres hum 220ft underground.

This is the Iron Mountain storage facility in Butler County, one of the most secure sites on the planet, where the US Government and leading companies keep vital data protected from earthquake, fire and theft.

The "mine" is huge, stretching two miles by three miles, and is a self-sufficient underground city with its own water treatment plant, power back-up system and fire brigade.

In the cavernous vaults, various branches of the US Government and security agencies store millions of high-security files, both paper and digital. It is where recording companies keep the master tapes of thousands of hits, from Elvis Presley onwards, and in a special low-temperature, low-humidity vault lies the Corbis archive – more than 11 million photographs and documents dating back to the early 19th century.

The centre is a vast labyrinth of limestone tunnels, with 110 storage vaults the size of football pitches behind heavy doors set into the rock. It is cool and dark, perfect for storing documents at controlled temperatures. Security is so tight that the federal Government classifies it just one level below the White House and Pentagon.

The 145-acre complex, where 2,700 people work every day far from sunlight, is an invaluable record of the past but also a storage centre for the future. It is run by Iron Mountain, a global data protection and storage services company, which has taken the mine's unique characteristics and turned it into the jewel of its $3.1 billion (£2 billion) global business managing information for companies and organisations that are required by law to hold on to important records.

The underground centre opened in the 1950s storing paper documents as fears of nuclear disaster prompted businesses to seek secure locations. There were even dormitories for company executives in case of missile attacks from Russia.

Now the emphasis is on digital storage, where companies need to keep important files for back-up and to retrieve in case of litigation.The recent turmoil in the financial sector has reinforced the need for such storage systems.

Iron Mountain has 10 billion e-mails in its trust worldwide and takes in about 8 million for archiving each day. Business is brisk – the amount of digital data it stores grew by 600 per cent between 2005 and 2008. At Butler County, 400,000 PCs and 20,000 servers are backed up daily.

The site has become more important for businesses in the wake of the 2001 terror attacks and more recently Hurricane Katrina, which forced companies to reassess how they store important documents. The hotel group Marriott International recently set up a disaster recovery system in one of the vaults, from which it can retrieve data in less than four hours.

Chuck Doughty, Iron Mountain's vice-president of engineering at the Butler County centre, has worked here for nearly four decades and is used to the lack of daylight. He has helped turn the facility from an old-style storage centre for paper into a hi-tech data facility using cutting-edge, environmentally friendly techniques to cool the servers.

He surveys the network operation centre, where staff monitor a huge bank of computers analysing conditions within the vaults and the servers. The charts show that the environment has not fluctuated by more than a degree in 20 years, he says. This part of Pennsylvania is zero-rated for earthquake activity, and Mr Doughty is investigating using geothermal cooling to make the data centres even more stable.

The mine's stability attracted Corbis, Bill Gates's photographic archive, which set up its preservation centre in 2002. The Corbis Film Preservation Facility sits inside a 10,000 sq ft underground chamber which is designed to be environmentally controlled at minus 20C with a relative humidity of 35 per cent. These conditions preserve unstable acetate film, fading dyes in colour transparencies and negatives, and photographs, enclosures and indexing systems.

The Bettmann Archive, known as the world’s photo album, is a collection of more than 11 million negatives and prints from the 19th and 20th centuries.

Beginning as the contents of two steamer trunks that Otto Bettmann took with him as he left Nazi Germany in 1935 for the United States, the collection grew over the years with the eventual merger with the news photo library of United Press International. The collection includes such iconic news images as the Wright Brothers in flight, Albert Einstein sticking out his tongue and Rosa Parks refusing to leave her bus seat in Montgomery, Alabama, in 1955.

Back at the surface in Butler County, workers file out of the mine at the end of the day. Only the hundreds of cars in the car park betray the presence of this modern mine of information underneath the ground.

Explore Tech & Web

• Personal Tech

• The Web

• Gadgets & Gaming