Gustav Klimt's "Portrait of a Lady" disappeared from an art gallery in northern Italy 23 years ago. It was presumed stolen.

The location of the masterpiece has been one of the art world's biggest mysteries ever since — only to be solved after discovering a secret door in the gallery's walls.

In December, a gardener at the Ricci Oddi Modern Art Gallery in Piacenza, Italy, found a painting hiding in a plastic bag within an external wall while clearing away ivy. Experts confirmed Friday that that painting is the long-lost Klimt work.

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"I'd like to say with no lack of emotion that I can announce the painting is authentic," Piacenza Prosecutor Ornella Chicca told reporters Friday.

According to BBC News, "Portrait of a Lady" is valued at $66 million. The gallery plans to consult with experts to run further tests on the piece to uncover more of its mysteries — including, hopefully, who stole it. It will then go back on display at the gallery.

The gallery said in a press release Friday it wants the "gift" of the painting to reach as many people as possible. It plans to work with various institutions to fully tell the story of the work since it was purchased almost 100 years ago.

Police officers stand beside Gustav Klimt's "Portrait of a Lady," which was found last December at an art gallery, during a press conference in Piacenza, Italy, Friday, Jan. 17, 2020. AP Photo/Antonio Calanni

Klimt finished "Portrait of a Lady" in 1917, the year before he died. It was acquired by the Ricci Oddi gallery in 1925, and art lovers have been searching for the iconic work since it went missing on February 22, 1997 as the museum prepped for a special exhibit, BBC reports.

The work was the subject of the press the year before it went missing, after an art student discovered it was painted on top of another Klimt work: "Portrait of a Young Lady," not seen since 1912. Her theory was confirmed following X-ray testing by the gallery.

The discovery of the painting inside the gallery's wall suggests it may have never even left the building all those years ago.

New York art historian and dealer Robert Simon suggested perhaps the painting was stashed in the gallery wall and never sold because it was too famous and well-documented. Selling it may have been to risky and "impractical," he told "CBS This Morning: Saturday."

According to Reuters, the piece was on the list of the most valuable artworks missing in Italy, second only to a work by Caravaggio stolen from a church in Sicily in 1969.