Letterlocking
There was a cool article on this subject a few weeks ago: <a href="https://www.npr.org/2021/03/02/972607811/reading-a-letter-thats-been-sealed-for-more-than-300-years-without-opening-it" rel="nofollow">https://www.npr.org/2021/03/02/972607811/reading-a-letter-th...</a><p>Basically, opening centuries-old letters that are sealed with this technique is usually a destructive process: you might end up rendering some portions of the letter unreadable.<p>As such, many of these letters have never been opened! They might contain interesting things, but we have no idea.<p>Some researchers figured out a way to "unfold" X-rays of these intricately-locked letters, to render the letter legible without having to actually open it! It's a pretty cool technique.<p>The underlying paper is here: <a href="https://www.nature.com/articles/s41467-021-21326-w" rel="nofollow">https://www.nature.com/articles/s41467-021-21326-w</a>