When curators and conservators examined Leonardo's The Virgin of the Rocks, they hoped to find an underdrawing. What they did not expect to find was a completely different design, hidden under the paint.
Conservators at the Gallery used a technique called infrared reflectography to look through the layers of paint and reveal details of the preliminary drawings underneath.
The first part of the painting they focused on was the Virgin's head, but what sprang out were the face and hand of another figure. Leonardo had evidently started on one picture, and then abandoned it for the existing one.