They're under the sea, they're on the internet, and at the moment you can buy a kit for one in a shop near you.

They are time capsules, and as the clock ticks nearer to the next century, many people are planning to leave a record of their lives for future generations.

Blue Peter's time capsule is probably the best known in Britain and the lead-lined box, buried in 1971, is still intact, ready to be dug up by presenters John Noakes, Valerie Singleton and Peter Purvis in the New Year.

And as we blast off or the new millennium, new capsules are planned for New Zealand, Antarctica and outer space.

But preserving things from our era for posterity does take a bit of care.

Photos, videos and computer discs degrade after 20 years.

And there is no point bothering with coins or stamps, as these are preserved anyway in archives.

So try to choose items that reflect 90s Britain, for example a National Lottery mug, a yo-yo and an empty designer drink bottle.

Professor Brian Durrans, of the Museum Of Mankind and co-founder of the International Time Capsule Society, told the BBC: "You could put all sorts of embarrassing personal secrets in the capsule, secret stuff that you can't go and tell people about now.

"Consider the kind of information that would not otherwise be preserved and that maybe historians of the future will be grateful to you for."

And perhaps most of all, remember where you put it.

Ten thousand capsules have been buried over the last century, but most have been lost.

The International Time Capsule Society has a list of the most wanted missing capsules including one lost under Blackpool Tower and a capsule stolen from the US bicentennial celebrations in 1976.

In the UK, an association of 3,000 men is looking to preserve a little piece of history for the next 1,000 years by allowing just two people in every generation to know the whereabouts of a time capsule in Northamptonshire.

The stainless steel capsule belonging to the Gretton Association of Stickholders, Tin and Lightermen has been designed to last until the time comes for them to be unearthed in the year 3000.

Association president Ralph Binder said: "We have got to try to keep the burial site secret in case it gets robbed.

"Hence just two of us know of the site and the knowledge will be handed down from generation to generation."

The capsule, which contains the group's symbols of a mini walking stick, a tin and a lighter, will also include photographs and a list of the society's 3,000 current members - written on parchment to ensure it survives.

Business opportunity

The term "time capsule" was coined in 1938 at the New York World's Fair. That capsule, designed for long-term preservation - 5,000 years - was sunk into a special "time well" dug into the Earth.

The glass-lined, hydrogen-filled copper-alloy cylinder contained 1,000 photos, a can opener, a man's hat, safety pins and some contemporary sheet music.

Interest in time capsules typically grows in anniversary years and the internet has proven a fertile ground for businesses hoping to make money on the craze for millennium capsules. Many internet companies are selling capsules ranging from $20 to $750.

One web page promises virtual immortality: "Leave your mark on the future. Be a part of history."