NEW YORK, NY.- In Pablo Picasso’s 1900 painting “Le Moulin de la Galette,” revelers sporting dresses or top hats appear to be drinking, dancing and chatting. Beneath the partyers, under layers of paint, there is a hidden dog that the artist seems to have hastily painted over.

For decades, the dog went unnoticed. But recent research and extensive restoration of the painting for an exhibition revealed an auburn-coated King Charles spaniel with a red bow.

The painting, which is on display through Aug. 6 at the Guggenheim Museum in New York, is part of the 10-piece exhibition “Young Picasso in Paris,” which features some of the Spanish artist’s early work when he was living in France.

Before the exhibition, the Guggenheim, working in collaboration with the Metropolitan Museum of Art in New York and the National Gallery of Art in Washington, restored the painting, removing grime and varnish. The treatment revealed subtleties — such as the brushwork, color palette and spatial definition — that had gone unnoticed in the painting. Then, technical imaging unveiled an earlier version of the painting that included the lap dog in the foreground.

Julie Barten, a painting conservator at the Guggenheim, said in an interview that the painting had undergone an entire year of treatment. That process, she said, was “critical to gaining a better understanding of this picture” and to revealing the “quick strokes he used to obliterate the dog.”

In hindsight, Barten said, she realized that in an X-ray of the painting “you can actually sort of make out a dog.”

“We could certainly make out that there was something there,” Barten said. “But we couldn’t say what it was at the time.”

Megan Fontanella, a curator at the Guggenheim, said that it was a “surprise and delight” to discover the dog hidden in the painting.

“When we embark on analysis of a picture,” she said, “we don’t often know we’re going to find something so interesting, enticing as a dog.”

Throughout the day on Tuesday, visitors at the museum stopped in front of the painting to take pictures of it or to pose beside it. Some knew about the dog and stopped to look for it. Others, completely oblivious to Picasso’s hidden gem, walked past the painting without looking at it. One woman sat on a cushioned bench with her back to the painting, as she scrolled on her phone.

Krystal Lauk, 37, who was visiting from San Francisco, did not initially spot the dog in the painting.

When she learned about the hidden pup, it took her a moment to find it.

“Is it the blob in the foreground?” Lauk asked.

Anna Beatriz, 27, who was visiting New York from Brazil, said that she didn’t know about the dog, and that she was at the Guggenheim because she has an interest in architecture. But when she learned about the dog, she promptly gathered friends to see if they could spot it.

While the discovery of the dog came as a surprise to some museumgoers, art experts say that it was not uncommon for Picasso to leave behind such Easter eggs.

“We know that Picasso was doing this in his earlier work," Fontanella said. “He often leaves these kind of vestiges of earlier compositions.”

A hidden arm, for example, was uncovered a few years ago in Picasso’s “La Miséreuse accroupie.”

“It was really a part of his process to be constantly transforming one thing into another and often leaving clues about what lay underneath,” Barten said.

There are few clues, however, about why Picasso ultimately decided to do away with the dog.

Tom Williams, an art history instructor at Belmont University in Nashville, said that “it’s hard to imagine this particular painting with a dog in the foreground.”

“I’m not sure a dog, and particularly a lap dog, makes sense in the dark, uneasy and erotically charged atmosphere that Picasso conjured up so brilliantly in this picture,” Williams said.

But at the Guggenheim on Tuesday, Lauk, the visitor from San Francisco, wondered why Picasso had painted over the spaniel.

“Maybe,” she said, “he wasn’t feeling the dog.”

This article originally appeared in The New York Times.