A legal deposit library is one in which all printed materials are required to be deposited. The United States requires copies be sent to the US Copyright Office, but only if you want a copyright, and they don't retain materials forever. In the UK, there are six legal deposit libraries, but as of now, the only one a publisher is legally required to sent materials to is the British Library, and they keep it forever. As you can imagine, this collection is massive, requiring off-site warehouses. It's not easy to get a library card to do research among all those materials, but when an item is requested, they move heaven and earth to get it for you. This requires giant high-speed robots and an impressive archiving system that has evolved over time. It will have to evolve further, as they are now taking all UK materials published online as well. Tom Scott gives us an overview, and ends with some lucky timing.