Below the Medici Chapels in Florence, Italy are tombs designed by Michelangelo for the families of his patrons. But back in 1975, while work was underway to build a new exit for the space, conservationists discovered that underneath the plaster on the walls was some early 16th century graffiti. Specifically, there were charcoal and sanguine sketches made by Michelangelo. Later this month, the corridor—just 33 feet long, 10 feet wide, and 8 feet tall—will be open for public viewing.
From CNN:
It is believed that the artist hid in the claustrophobic space for several weeks in 1530 when pope Clement VII – a member of the Medici family, who had recently returned to power in Florence, having been kicked out by a republican government for whom Michelangelo had worked – ordered his death. The death sentence was rescinded after two months, and Michelangelo returned to work in Florence, before moving to Rome four years later.
It's believed the drawings are sketches for future works, including the legs of one of the statues in the New Sacristy.