NPR:

Math teacher Sherry Read’s classroom is a total mess. The students are gone for the summer, and light fixtures dangle from the ceiling. The floor has a layer of dust. Down the hallway, workers make a racket while they renovate the school, which dates back to the 1890s. They’re working in what has become an archaeological site.

A construction crew at the Oklahoma City school made a startling discovery earlier this month. They found old chalkboards with class lessons that were written almost a century ago, and chalk drawings still in remarkably good condition. So Read doesn’t mind the mess. In fact, she’s amazed.

Related: Math Forum and Connected Math.