Buried treasure: beneath Picasso's Rue de Montmartre (left) lies the first picture the artist painted after arriving in Paris. X-rays revealed A Hidden Picasso (below, left), which has now been digitally enhanced to display its original colours. Credit: PHOTO: DON MEYER/IMAGES: SFMOMA

For almost a century, Pablo Picasso's early painting Rue de Montmartre held a surprising secret. But a few years ago, X-ray analysis showed that beneath its restrained portrayal of some hunched-up figures there lurked a very different painting of the decadent side of Parisian nightlife.

Now William Shank, former chief conservator at the San Francisco Museum of Modern Art, has digitally recreated the hidden work in all its colourful glory. Named simply A Hidden Picasso, it is being exhibited for the first time this month at the Guggenheim Museum Bilbao in Spain, to coincide with an international meeting of art conservationists taking place in the city.

Shank began with the shadowy, black-and-white image revealed by the X-rays — the method used routinely by conservators to see through thin layers of paint. He then identified the painting's original colours, using cameras to peer through the networks of tiny cracks in Rue de Montmartre and computer imaging techniques to build up an image of what lies beneath it.

Experts date the uncovered painting to 1900, which would make it the first work that the artist painted after arriving in Paris from Barcelona at the age of 19 — giving the elusive work special significance for art historians.