A recent cleaning of a painting in Madrid's Prado has unveiled some of the mystery surrounding Leonardo da Vinci's Mona Lisa (ca. 1503-06).

Previously thought to be a later and inferior copy, the restored painting has been determined to be the earliest copy of da Vinci's masterpiece, and importantly, created by a key pupil at the same time as the original.

Restorers stripped away layers of black paint added in the 18th century to expose a beautiful Tuscan landscape behind the figure, who is thought to be Lisa Gherardini, the wife of the Florentine cloth merchant Francesco del Giocondo.

The radiant young woman who has intrigued viewers for centuries in the original, which hangs ibehind bullet-proof glass in the Louvre, is seen clearly in the pupil's replica.

Details of her clothing, chair, and smiling face are sharper than in the unrestored original which has been determined to be too fragile to clean.

The news has been hailed as one of the most important recent art discoveries. The Art Newspaper noted, "This sensational find will transform our understanding of the world’s most famous picture."

Once fully conserved, the replica will go on view at the Prado around mid-February before heading to the Louvre in Paris, as part of its exhibition “Leonardo’s Last Masterpiece: The Sainte Anne” (March 29-June 25, 2012).