An intern at the Belgian Royal Library has discovered a hidden engraving on the back of an engraving of one of Pieter Bruegel the Elder’s works.

Twenty-three-year-old Margaux Nogues was cleaning The Stone Operation , an engraving of Bruegel’s work dating from 1559 made by Hieronymus de Cock,  during her traineeship when she removed a piece of cardboard backing from the more than 400-year-old engraving, the institute said in a press release.

‘When I took it off, I discovered another work on the back: a second engraving by Brueghel,’ Nogues told Belgian broadcaster RTFB.

The second engraving was of Brueghel’s work Patience, a 1557 work depicting the personification of the virtue patience, who is surrounded by monsters that represent evil and sin. Born in what is now North Brabant, Pieter Bruegel the Elder is considered the most significant artist of the Dutch and Flemish Renaissance.

Both engravings are thought to be the work of De Cock. The museum describes the find as ‘very unusual,’ as the type of printing used was uncommon in the 16th century.