Room of the wooden boards, vault frescos depicting interwoven plants with fruits and monochromes of roots and rocks, by Leonardo da Vinci and collaborators, Sforza Castle, Milan. Italy, 15th century. (De Agostini/Getty Images)

A 15th-century Leonardo da Vinci mural has been discovered in a Milan castle.

Traces of the mural were found in Sforza Castle. The mural, which depicts a canopy of mulberry trees, has been hidden underneath multiple layers of whitewash - sometimes up to 17 layers - for up to 500 years, according to the Opficio delle Pietre Dure, a restoration institute in Florence. The organization has been researching the mural for two years, and discovered it while restoring the Sala delle Asse, the Room of Planks. Da Vinci's mural is expected to be fully restored by May 1, 2015, in time for the inauguration day of the Universal Exhibition.

The Italian Renaissance master worked on the mural from April to September of 1498. Ludovico Sforza, the duke of Milan, had hired da Vinci and a few others to decorate the castle.

One who sees the mural "is completely surrounded by a canopy of branches, identified as mulberry, interwoven and supported by powerful drums from enormous tree roots," it says on the Sala delle Asse's restoration website. "The branches, crossing the ceiling with knotted string, create a geometric pattern repeated placed next to the theme of the famous da Vinci knots."