When the printing press made its debut in Europe in the 15th century, thousands of old texts were ‘recycled’ for use as binding material for newer books. In 2015, researchers at Leiden University started using an x-ray technique to reveal these texts. Some are up to 1,300 years old. https://www.universiteitleiden.nl/en/news/2015/12/digitally-leafing-through-invisible-books