If you're in the UK and you publish a book, magazine, or newspaper, then by law you have to send a copy here.

A lot of countries around the world have similar laws and similar libraries.

The rules for what must be sent here were pretty simple to understand...

until the end of the 20th century, when the web came along, and suddenly, books and publishing weren't quite as easy to define.

In the print world, we collect everything.

So anything you can imagine, whether that is an NHS information leaflet, local magazine, or the most expensive journal that you can imagine.

Or those beautiful art books that cost thousands of pounds.

Those kids' magazines that you see on the shelves in Tesco's with the plastic toys on. We collect all of those. It's everything.

Down in the basement here, there's obviously a huge amount of storage. But it's not enough.

And here in London, land is expensive and construction is difficult.

So while there are millions of books stored here, they're only the ones that are the most requested.

The rest of them are with the robots up north.

The library collection in total is around 170 million items.

The vast majority of that is "legal deposit".

Often publishers know legal deposit and will send them in.

More of a challenge can be very small publishers, or people who don't necessarily think of themselves as publishers.

So we put a lot of effort into reaching out to authors, creators who self publish, all those kinds of organisations to say that the work is really important to be saved and preserved.

In total, there are more than 700km of shelves, and most of them are stacked up here, in one of the colossal low-oxygen storage buildings in Boston Spa in Yorkshire.

I was not allowed to attach a camera to one of these robots, they are far too important.

The library's using up those shelves at about 8km per year, so they've just started construction on their next storage building, which should be good for a few decades.

Lots of the stuff that gets sent out free tends to be ephemera, and it will be lost over time.

A great example is a recent exhibition up in the National Library of Scotland, all about the information that was provided at the very beginning of the AIDS epidemic.

That material, if you go out to somebody on the street and you ask them, "Should we be collecting this?"

"No, of course we shouldn't."

But we're able to learn so much from that.

We wouldn't have that exhibition if we hadn't collected that free material that came through people's letterboxes.

If someone in London wants a lesser-used book from these stacks, they need to give 48 hours' notice. It'll get picked out by these robots.

They up to 50 kilometers an hour.

If you can hear wind rustling on my mic, it's because they shove air out the way as they travel through the stacks.

And they automatically store items that are used more often close to the front.

The books are then put in a truck south, to arrive in the reading rooms a couple of working days later.

More than a thousand items go back and forth each day. But publication means something different now.

Not only are there books that are only being publishedas digital files, but there's the web.

Legal deposit has expanded to include publishing on the web, as long as we can identify that as being within the UK.

So if someone's creating a blog, or creating their own website, we will try to collect that.

The UK Web Archive has actually become one of the largest parts of the collection.

Billions of files, about one and a half petabytes of data!

A digital newspaper, for example, is very different from a print newspaper.

So we have to make sure we collect both because the editorial intent of both is very different.

And in addition to that, there are emerging formats.

You can read books which change as you move the iPad around. You can read books that change depending on your location.

How do we capture that today, to make it available for researchers in 10 years', 50 years' time?

So it's quite difficult to keep up with those changes.

I've used the British Library for research a lot of times.

The books you request can't be borrowed and taken away.

They have to stay with the researcher, at a desk inside one of the reading rooms, returned after they're done.

This is a library of last resort.

If you want to get a library card here, there will be a short interview about your research and why you can't just pull the books or material you want from your local or university library.

The importance of legal deposit not being selective, and being everything, is: we can't decide today what's going to be important in 50 years' time.

We want everything, because we don't know what will be important.

I cannot overstate just how useful it is to be able to track down things that never made it online, or to research out of print, forgotten books where there are no other copies available,

or to scan through every issue of an obscure local newspaper to track down one reference.

This is the raw text of history as it happened and someone has to keep it preserved for the future.

How's that for timing?

[camera op] "That was brilliant. Fantastic."

I wish I'd done that deliberately!