"The Grolier Codex, an ancient document that is among the rarest books in the world, has been regarded with skepticism since it was reportedly unearthed by looters from a cave in Chiapas, Mexico, in the 1960s. But a meticulous new study of the codex has yielded a startling conclusion: The codex is both genuine and likely the most ancient of all surviving manuscripts from ancient America." Source: Science Daily.
According to Trafficking Culture, "The Grolier Codex (sometimes known as the Saenz Codex) consists of eleven fig bark sheets, stuccoed and painted on one side, that are allegedly from a twenty one page Maya book ... The lower portion of all pages is badly water damaged. Eight blank sheets of fig bark paper, all unstuccoed, are associated with the codex. One of these sheets was carbon dated to AD 1230 ± 130 ... For some time there was confusion about the whereabouts of the Grolier Codex, but it is currently known to be housed at the Museo Nacional de Antropología in Mexico City and is not on public display..."
The codex has a convoluted history. According to the story, which may or may not be true, a Mexican collector purchased the artifact from some looters after flying in secret to meet them at a remote airstrip, the location of which they tried to conceal. The codex got its name because it was later displayed at the Grolier Club in New York. The history after that is very murky and I suppose a conspiracy theorist could make a lot out of the fact that the codex is not on public display.