Yahoo! suffered a denial-of-service attack Monday morning -- knocking one of the Web's most popular sites offline for approximately three hours. According to a Yahoo! spokesperson, at 10:30 a.m. Pacific Standard Time Monday the company's routers "experienced a coordinated, distributed denial-of-service attack at one of its California data centers, which caused intermittent inability to access some, but not all Yahoo! services." Yahoo! is believed to be the biggest site ever to be forced offline by a denial-of-service attack. The site was back online by 1:30 p.m. PST. According to Yahoo!, all user information and data is intact. The attackers were not able to access or change any private information, such as passwords. Yahoo!'s email service was not affected by the attack. Yahoo! delivers an average 465 million page views per day. It is the second most visited site on the Internet after America Online, drawing more than 42 million unique visitors a month, according to research firm Media Metrix. Yahoo! experiences attempted denial-of-service attacks on a fairly regular basis, according to the spokesperson, but it is normally able to override the problem by re-routing resources. This was the first time the site was forced offline by such an attack. "Today was pretty unusual," said Yahoo's spokesperson. "It happened so quickly and intensely they couldn't redirect all their traffic." By Monday evening the site had filters in place to block the attack, and the network was running at full scale. It was not yet clear if the denial-of-service attack was continuing. At this time, Yahoo! says it does not know who is responsible for taking the site down. Although Yahoo! refuses to estimate how much Monday's attack cost it in lost revenue, analysts estimate the losses will run in to the millions of dollars. "You're talking about one of the biggest sites on the Web going down in the middle of the business day," said Malcolm Maclachlan, media and e-commerce analyst for International Data Corporation. "That's pretty significant." Maclachlan says the approaching Valentine's Day holiday will likely make the e-commerce losses on Yahoo!'s shopping site even more dramatic. Maclachlan estimates Yahoo! typically does "millions of shopping dollars in a four-hour period." Harry Fenik, executive vice president of Zona Research believes the real effect of the outage will likely be more psychological than financial. Yahoo! has prided itself on its reputation as an "unhackable" site. "It's losing prestige," Fenik said. "Companies like Yahoo! are saying to companies 'Use us. Bank your life on us.' Then they go away for half a day. This doesn't go over so well." One of Yahoo!'s investors, Softbank, is also a major investor in ZDNet Margaret Kane, ZDNet News US, contributed to this report What do you think? Tell the Mailroom. And read what others have said.