Perhaps the most famous single piece of sculpture in all the world, the Pietá was completed by Michelangelo in 1499 when the sculptor was just 24 years old. It is the only work he ever signed. Responding to critics who said he didn't actually create it, Michelangelo cut the words, MICHAEL. ANGELUS. BONAROTUS. FLORENT. FACIEBAT" (Michelangelo Buonarroti of Florence Created This). After being attacked by a mentally disturbed man wielding a hammer on May 21, 1972, the sculpture was repaired and now sits behind bullet-proof acrylic glass.