The Web site of Amazon.com, the Web's biggest retailer, was down and unavailable for two hours today.

On an Amazon discussion forum for sellers, a post entitled "Amazon Un-Planned Event (06/06)" says that the company is "currently investigating an issue that has impacted the availability of the Amazon.com website."

"Engineers are actively engaged in resolving this issue and we will provide an update once the issue is resolved," the post says. "We appreciate your patience during this time."

The post is dated 10:37 West Coast time. An Amazon spokesperson said, "Amazon's systems are very complex and on rare occasions, despite our best efforts they may experience problems. We work to minimize any disruption and to get the site back as quickly as possible."

Amazon's S3 service, which many Web sites use for back-end functions like serving up large amounts of data, appears to be operating normally.

Outages among major Web companies like this are rare nowadays. Back in 1999, eBay went down for 22 hours, costing it millions in revenue.