Wine Therapy is a web bulletin board for serious wine geeks. It's been active since 2000, and its database of back posts and comments is a wealth of information: tasting notes, restaurant recommendations, stories and so on. Late last year someone hacked the board software, got administrative privileges and deleted the database. There was no backup.

Of course the board's owner should have been making backups all along, but he has been very sick for the past year and wasn't able to. And the Internet Archive has been only somewhat helpful.

More and more, information we rely on – either created by us or by others – is out of our control. It's out there on the internet, on someone else's website and being cared for by someone else. We use those websites, sometimes daily, and don't even think about their reliability.

Bits and pieces of the web disappear all the time. It's called "link rot," and we're all used to it. A friend saved 65 links in 1999 when he planned a trip to Tuscany; only half of them still work today. In my own blog, essays and news articles and websites that I link to regularly disappear – sometimes within a few days of my linking to them.

It may be because of a site's policies – some newspapers only have a couple of weeks on their website – or it may be more random: Position papers disappear off a politician's website after he changes his mind on an issue, corporate literature disappears from the company's website after an embarrassment, etc. The ultimate link rot is "site death," where entire websites disappear: Olympic and World Cup events after the games are over, political candidates' websites after the elections are over, corporate websites after the funding runs out and so on.

Mostly, we ignore the problem. Sometimes I save a copy of a good recipe I find, or an article relevant to my research, but mostly I trust that whatever I want will be there next time. Were I planning a trip to Tuscany, I would rather search for relevant articles today than rely on a nine-year-old list anyway. Most of the time, link rot and site death aren't really a problem.

This is changing in a Web 2.0 world, with websites that are less about information and more about community. We help build these sites, with our posts or our comments. We visit them regularly and get to know others who also visit regularly. They become part of our socialization on the internet and the loss of them affects us differently, as Greatest Journal users discovered in January when their site died.

Few, if any, of the people who made Wine Therapy their home kept backup copies of their own posts and comments. I'm sure they didn't even think of it. I don't think of it, when I post to the various boards and blogs and forums I frequent. Of course I know better, but I think of these forums as extensions of my own computer – until they disappear.

As we rely on others to maintain our writings and our relationships, we lose control over their availability. Of course, we also lose control over their security, as MySpace users learned last month when a 17-GB file of half a million supposedly private photos was uploaded to a BitTorrent site.

In the early days of the web, I remember feeling giddy over the wealth of information out there and how easy it was to get to. "The internet is my hard drive," I told newbies. It's even more true today; I don't think I could write without so much information so easily accessible. But it's a pretty damned unreliable hard drive.

The internet is my hard drive, but only if my needs are immediate and my requirements can be satisfied inexactly. It was easy for me to search for information about the MySpace photo hack. And it will be easy to look up, and respond to, comments to this essay, both on Wired.com and on my own blog. Wired.com is a commercial venture, so there is advertising value in keeping everything accessible. My site is not at all commercial, but there is personal value in keeping everything accessible. By that analysis, all sites should be up on the internet forever, although that's certainly not true. What is true is that there's no way to predict what will disappear when.

Unfortunately, there's not much we can do about it. The security measures largely aren't in our hands. We can save copies of important web pages locally, and copies of anything important we post. The Internet Archive is remarkably valuable in saving bits and pieces of the internet. And recently, we've started seeing tools for archiving information and pages from social networking sites. But what's really important is the whole community, and we don't know which bits we want until they're no longer there.

And about Wine Therapy, I think it started in 2000. It might have been 2001. I can't check, because someone erased the archives.

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Bruce Schneier is CTO of BT Counterpane and author of Beyond Fear: Thinking Sensibly About Security in an Uncertain World. You can read more of his writings on his website.