Anna Patterson's last Internet search engine was so impressive that industry leader Google Inc 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 US$33 million in venture capital, the search engine plans to begin 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.

Costello's Irish heritage inspired Cuil's odd name. It was derived from a character named Finn McCuill in Celtic folklore.

Now, it's boasting time.

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

Patterson believes that is 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 one 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.

Search index's scope

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 websites, Patterson says Cuil's technology drills into the actual content of a page. And Cuil's results will be presented in a more magazine-like 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 is hoping to attract traffic by promising not to retain information about its users' search histories or surfing patterns - something that Google does, much to the consternation of privacy watchdogs.

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 was acquired by Microsoft Corp this month.

Even after investing hundreds of millions of dollars on search, both Microsoft and Yahoo Inc. have been losing ground to Google.

Through May, Google held a 62 per cent share of the US search market followed by Yahoo at 21 per cent and Microsoft at 8.5 per cent, according to comScore Inc.

Google has become so synonymous with Internet search that it may no longer matter how good Cuil or any other challenger is, said Gartner Inc 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 its 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 that Google has battled a general-purpose search engine created by its own alumni.