Pascal Cotte discovered a secret. He found a hidden drawing under a famous painting. The drawing shows how Leonardo da Vinci painted the Mona Lisa. It gives clues about the famous painting of the lady with the mysterious smile.

In 2004, Mr. Cotte got permission to scan the Mona Lisa. He used a camera that he invented. The camera took 1,650 pictures of the painting. Mr. Cotte spent more than 15 years studying all the images. He looked for hidden reflections under many layers of paint. Looking closely, Mr. Cotte saw “very fine details.” There were faint charcoal lines under the centuries-old paint.

The lines Mr. Cotte discovered are from a drawing technique called spolvero. An artist pricks tiny holes around a sketched outline. The artist rubs charcoal dust on the back of the sketch. Then the sketch is rubbed onto a canvas. The charcoal dust makes a simple copy of the work.

That’s what Mr. da Vinci did to create his Mona Lisa masterpiece. Until now, no one knew that he used the spolvero technique. Mr. da Vinci sketched first and painted later. “The spolvero on the forehead and on the hand betrays a complete under-drawing,” says Mr. Cotte.

God doesn’t need a fancy camera to see secrets. Jeremiah 33:3 says, “Call to me and I will answer you, and will tell you great and hidden things that you have not known.”

Mr. Cotte’s research showed other surprises. The images Mr. Cotte took of the painting reveal curls on top of the smiling lady’s smooth hairstyle. A squiggle that looks like a hairpin is hiding in the ringlets. The sketches show other changes to the woman’s costume and the chair she sits in.

No one knows why the Renaissance artist changed the sketch for his final painting. But seeing the changes shows that even great artists plan and practice.

The spolvero discovery raises art fans’ hopes for another mystery to be solved in the future. Is there an original paper sketch of the painting hiding somewhere?