This used to happen so often it made Twitter's "fail whale" famous. However, as Twitter's popularity has exploded and the hype surrounding it has become overbearing, the fledging company has gotten its act together in terms of keeping the site up.

Not so this morning. (Update below: It's a denial-of-service attack.)

As I type, Twitter has been down for just over an hour. From the company's status blog: "We are determining the cause and will provide an update shortly."

And, of course, when Twitter is unavailable people notice.

Writes Dan Frommer on Silicon Alley Insider: "Twitter is down! This seems to be happening more, recently, than in the last several months. What's going on, guys? It's going to be tough to become the next AT&T if you're dead at 9 a.m. on a Thursday."

The data doesn't support Frommer's suggestion that this is happening more often, although the contention does show how difficult it can be for a company to shake a bad reputation.

According to the Web monitoring company Pingdom, this is Twitter's first outage of more than 5 minutes duration since June 16. Over the past six months, the site has had just under eight hours of downtime for an uptime record of 99.8%.

Pingdom noted Twitter's improved uptime performance in a report released in February.

What makes the ongoing outage curious -- and perhaps troubling -- is that the site is completely inaccessible; no fail whale this time.

(Update, 10:25: Hugh Briss, owner of Twitter Image, adds detail to the extent of the outage: "Twitter has been down for close to an hour now and when I say down I don't mean Fail Whale "oops we're having a problem be back in a minute" down, I mean down as in even the API apps like TweetDeck aren't working.")

(Update, 10:55: From Twitter comes word they are under attack: "We are defending against a denial-of-service attack, and will update status again shortly.")

(Update, 11:20: Twitter says "the site is back up, but we are continuing to defend against and recover from this attack." I could not get on the site just now.)

(Update, 11:35: A commenter on Reddit contends that using Ruby on Rails may be why Twitter's off the track. "When people say Ruby on Rails doesn't scale, this is the kind of thing they're talking about. Twitter got much better uptime by switching their messaging back end from Ruby on Rails to Scala. If they want to be able to survive simple DoS attacks they need to move their front end away from Ruby too." ... Site's still down from where I sit.)

(Update, noon: Twitter's back.)

(Update, 12:10: Site is up but it won't let me tweet, which is akin to a bar being open without serving drinks.)

(Update, 12:40: Our long national nightmare is over: I can tweet again.)