Mental Floss:

The British Library is the largest library collection in the world, with more than 170 million items in its catalogue.

The collections are generated by the so-called “Mechanical Curator”, which randomly chooses images from public domain books from the 17th, 18th and 19th centuries. Better yet, every image links back to a PDF of the source book so you can determine the full context. The sheer volume of images means it’s impossible for the institution to organise them alone, and so part of the project is a request that users help identify and tag the content. Essentially, you get to contribute to the British Library’s curation.

Do not go to The British Library’s Flickr page unless you have a lot of free time. It’s an absolute time suck.