At the end of November in 1917 new chalkboards were installed on top of old ones in a school in Oklahoma City, leaving untouched what had been written and drawn on them. More than 97 years later, a few days ago in fact, contractors replaced both sets of blackboards with new whiteboards and digital "smartboards," in the process discovering writings and drawings and arithmetic problems unseen and untouched for almost a century. Some of it is quite remarkable, including a sort of circular multiplication table and handwriting that's quite beautifully done.
You can find an article about the discovery and an accompanying set of photos to click through on The Oklahoman newspaper's NewsOK website here. There's also an article in The Washington Post at this link.