The National Archives opened a treasure trove to genealogists and historians on Monday, releasing the 1940 national census in its entirety -- and doing so for the first time online.

By going onto 1940census.archives.gov, anyone with an Internet connection can plunge into 3.9 million digitally scanned pages of census records via a search function based on location.

In the coming months, an army of 300,000 volunteers, sifting electronically through every page, will build a database that will make it possible to search the website by name as well.

By law, in the United States, detailed information about individuals from a decennial census -- a precise snapshot of every household in every place in every state -- can only be made public 72 years after it is taken.

But experts say the 1940 census is particularly special, as it was taken as the United States struggled to climb out of the Great Depression as its entry into World War II loomed.

"We now have access to a street-level view of a country in the grips of a depression and on the brink of global war," said National Archives chief David Ferriero at a launch event in Washington.

For a growing number of Americans researching their own family trees, "it's almost like Christmas," said Ferriero, who looked up his own Italian immigrant grandparents using their street address in a Massachusetts mill town.

Some 120,000 enumerators collected detailed information about 132,163,569 people -- more than 21 million of whom are still alive today -- for the 1940 census, Ferriero said.

It was the 16th decennial census since the founding of the United States, but the first to ask a raft of socio-economic questions designed to measure the impact of the Great Depression.

Researchers can go back in time to discover the names, ages and relationships of everyone in a given household, what those people did for a living, what they earned, even whether they had a radio or a flush toilet.

The census takers also asked respondents where they'd been living five years earlier -- a question that could help pinpoint Depression era migration patterns.

Previous censuses were released on microfilm by the National Archives, which is hosting a genealogical fair in Washington on April 18-19.