As a Pablo Picasso 1901 masterpiece in Washington DC reveals a portrait of an unknown man hidden under its surface, we look at five great works of art long lost to the public

This is not the first time that a hidden painting has been discovered beneath one of Picasso's works.

Experts found a portrait of a man with a moustache beneath his 1904 painting Woman Ironing, which is held by the Guggenheim Museum in New York.