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To use the new service, you can either go to the Google News Archive, or search Google News and switch to the “timeline” view. When a story is viewable in its original format, the search result will list “Google News Archive” next to the name of the source.
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It’s not clear how much content has been indexed so far, but searches for huge events like “Kennedy Assassination” and “Pearl Harbor” don’t seem to yield any digital archives yet, so I’d assume the index is still fairly small. As an example of a search that does show some results, Google suggests “Americans walk on the moon” which brings up original articles from a number of newspapers on July 21st, 1969.