Da Vinci sketch found under painting!

A new sketch by Da Vinci has recently been found when his masterpiece "the virgin of the rocks" was being studied. I wonder how many more paintings are hidden under his paintings? I heard they were cleaning a ceiling in a cathedral only to find one of his paintings under all the dirt.

Source: New Sketch found

There is nothing special in that, I think that it is normal for a artist to start doing something and then change his mind and make something different. There are many cases of such findings in paintings, in sculpture it is impossible to see what was there before.

Also, one of the most important Portuguese paintings was found when someone noticed that the boards that were being used in a scaffolding had something painted on them.

Overpainting was fairly common, as was painting over another sketch. Materials in those days were extremely expensive, and if you had something that wasn't working out, you didn't throw it away... you just reused it. If you didn't like a painting after you'd finished it, you'd just gesso over it and do a new one.

It would be interesting to see what else is hidden under other paintings.