"We Came In Peace For All Mankind" - And Left Messages

"The Untold Story: How One Small Disc Carried One Giant Message for Mankind" tells about a silicone disk (about 1 1/2 inches across, or roughly 4 centimeters, or roughly the size of an American dollar coin), left by the Apollo 11 crew at Tranquility Base, and now nearly forgotten.

There's more detail at collectgSPACE.com.

For me, the disk itself isn't particularly interesting, apart from the technology and art involved in making it: it's the messages on the disk.