“In the vastness of the web, what is the sampling of stuff that we can pull together that demonstrates what’s going on now?” said John Fenn, the head of research and programs at the American Folklife Center. He is also one of about 80 recommending officers, who make suggestions for the library’s archive — in Mr. Fenn’s case, for the Web Cultures collection. (It is one of several thematic groupings in the archive, along with the Webcomics collection, American Music Creators and dozens more.)

“It’s like whack-a-mole,” said Gina Jones, a digital projects coordinator on the team.

The criteria for selection typically used by print archivists — value to future scholars, uniqueness of the material — still apply to the web archivists, though the high extinction rate of digital matter factors into decision making. One of the most recent acquisitions is the recently defunct Design Sponge, an interior decorating website that ran for 15 years. (Though it will cease to exist as a website, every single blog post will be fully accessible through the Library’s web archive.)

The earliest material in the archive dates to the 2000 elections, when the web archive was still a pilot program. After the terrorist attacks of 9/11, when heart-rending memorials and fierce political debates played out online, the library recognized the need for an official digital record.

For years, collecting was keyed to major news events: the Iraq War, the 2004 elections. Then, around 2009, came a more continuing, expanded approach that sought to reflect the web in all its dizzying newness.