Anna Patterson's last Internet search engine was so impressive that industry leader Google bought the technology in 2004 to upgrade its...

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SAN FRANCISCO — Anna Patterson’s last Internet search engine was so impressive that industry leader Google bought the technology in 2004 to upgrade its own system.

She believes her latest invention is even more valuable — only this time it’s not for sale.

Patterson instead intends to upstage Google, which she quit in 2006 to develop a more comprehensive and efficient way to scour the Internet.

The end result is Cuil, pronounced “cool.” Backed by $33 million in venture capital, the search engine began processing requests for the first time Monday.

Cuil had kept a low profile while Patterson, her husband, Tom Costello, and two other former Google engineers — Russell Power and Louis Monier — searched for better ways to search.

Now, it’s boasting time.

For starters, Cuil’s search index spans 120 billion Web pages.

Patterson believes that’s at least three times the size of Google’s index, although there is no way to know for certain. Google stopped publicly quantifying its index’s breadth nearly three years ago when the catalog spanned 8.2 billion Web pages.

Cuil won’t divulge the formula it has developed to cover a wider swath of the Web with far fewer computers than Google. And Google isn’t ceding the point: Spokeswoman Katie Watson said her company still believes its index is the largest.

After getting inquiries about Cuil, Google asserted on its blog Friday that it regularly scans through 1 trillion unique Web links.

But Google said it doesn’t index them all because they either point to similar content or would diminish the quality of its search results in some other way. The posting didn’t quantify the size of Google’s index.

A search index’s scope is important because information, pictures and content can’t be found unless they’re stored in a database. But Cuil believes it will outshine Google in several other ways, including its method for identifying and displaying pertinent results.

Rather than trying to mimic Google’s method of ranking the quantity and quality of links to Web sites, Patterson says Cuil’s technology drills into the actual content of a page. And Cuil’s results will be presented in a more magazinelike format instead of just a vertical stack of Web links.

Cuil’s results are displayed with more photos spread horizontally across the page and include sidebars that can be clicked on to learn more about topics related to the original search request.

Finally, Cuil hopes to attract traffic by promising not to retain information about its users’ search histories or surfing patterns — something Google does, much to the consternation of privacy watchdogs.

After making so many bold promises, Cuil got off to a rocky start Monday as its computers were overwhelmed by curious Web surfers. As of late Monday afternoon, even simple search requests were still being greeted with this message: “No results because of high load.”

Cuil is just the latest in a long line of Google challengers.

The list includes swaggering startups like Teoma (whose technology became the backbone of Ask.com), Vivisimo, Snap, Mahalo and, most recently, Powerset, which Microsoft acquired this month.

Even after investing hundreds of millions of dollars on search, both Microsoft and Yahoo have been losing ground to Google. Through May, Google held a 62 percent share of the U.S. search market followed by Yahoo at 21 percent and Microsoft at 8.5 percent, according to comScore.

Google has become so synonymous with Internet search that it may no longer matter how good Cuil or any other challenger is, said Gartner analyst Allen Weiner.

“Search has become as much about branding as anything else,” Weiner said. “I doubt [Cuil] will be keeping anyone at Google awake at night.”

Google welcomed Cuil to the fray with its usual mantra about rivals. “Having great competitors is a huge benefit to us and everyone in the search space,” Watson said. “It makes us all work harder, and at the end of the day our users benefit from that.”

But this will be the first time Google has battled a general-purpose search engine created by its own alumni.

Patterson joined Google in 2004 after she built and sold Recall, a search index that probed old Web sites for the Internet Archive. She and Power worked on the same team at Google.

Although Monier also worked for Google for a short time, he is best known as the former chief technology officer of AltaVista, considered the best search engine before Google.

The trio of ex-Googlers are teaming up with Patterson’s husband, Costello, who built a search engine called Xift in the late 1990s. He later joined IBM, where he worked on an “analytic engine” called WebFountain.

Costello’s Irish heritage inspired Cuil’s odd name. It was derived from a Celtic folk character, Finn McCuill.