Portraits of two Elizabethan courtiers, it seems, were painted over Catholic religious paintings.
The National Portrait Gallery displays a portrait of Sir Francis Walsingham, spymaster for Queen Elizabeth, with an x-ray view of a Virgin and Child underneath. (photo)
Also discovered was a Flagellation of Christ under a portrait of Thomas Sackville, Lord Treasurer for Elizabeth. The article lists several other paintings from other museums where x-rays have discovered hidden paintings.