On Wednesday, November 15th, Leonardo da Vinci’s Salvator Mundi (circa 1500) sold for $400 million ($450 million with fees) after 19 minutes of bidding, making it the most expensive piece ever purchased at auction, and likely the most expensive artwork ever sold.

Since October 13th, ’s Christ portrait Salvator Mundi, or “Savior of the World,” has traveled the globe from Hong Kong to San Francisco, and has been celebrated as the “greatest artistic rediscovery of the 21st century” by Christie’s. Now it’s back in New York, where, on November 15th, it will go on offer in the house’s Post-War and Contemporary evening auction, where it is estimated to sell for $100 million.

Fewer than 20 works by Leonardo are known to exist in the world, and the last authentic original was discovered in 1909, so the arrival of this previously unpublished painting by the Renaissance master is considered a seismic Leonardo event. The painting was presented as “discovered” at a 2011 exhibition at London’s National Gallery, to great international fanfare. Christie’s chairman Loïc Gouzer said in a statement that “the opportunity to bring this masterpiece to the market is an honor that comes around once in a lifetime.”

But how did the auction house come to the estimate of $100 million? Works of art that have sold for more than $100 million at auction are rare, but we’ve seen a number of them in recent years. An untitled painting sold for $110.5 million in May; in 2015, ’s Women of Algiers sold for $179 million and an nude sold for $170 million; and in 2013, an car crash painting sold for $105.4 million.

No painting has ever crossed the $100 million mark at auction, though the French businessman Eric de Rothschild sold a pair of portraits by in a private sale in 2015 to the Rijksmuseum and the Louvre for nearly $180 million, or $90 million a piece. While the record-breaking contemporary artists that top the market obviously have admirers and appeal, Leonardo is still considered by many people to be the most revered artist of all time. And, therefore, a lot of people think it should follow that his work sells for the highest prices ever recorded.

Christie’s Old Masters specialist Alan Wintermute explained by email that it wasn’t easy to come up with an estimate. “There are no direct comparables for a work by this artist and of this art historical significance, and the estimate is considerably higher than any Old Masters painting previously sold at auction,” said Wintermute, who points out that the current auction record for an Old Master is £50 million (or about $65.5 million), paid for a work in London in 2001.

“At this point there has been significant interest in the painting, and ultimately the market will dictate where the final price lands.” Christie’s has secured a third-party guarantee, which some media outlets have reported to be in the range of $100 million, though the auction house declined to discuss the amount. Wintermute added, “This work and artist transcend all collecting categories, and this is a work that appeals to all serious collectors regardless of their preferred era or medium.”

Old Master art dealer Robert Simon, a New York art dealer who once co-owned the work, oversaw its restoration, and co-authored a forthcoming book on it, said that if he were asked to sell it privately, “I’d ask $300 million, and being a pragmatic sort, I’d settle for $250 million.”

He said he came to this estimate after considering other major works of art that have sold both at auction and privately—such as the rumored private sale of a piece for about $250 million in 2012—and considering the prices of the few Leonardos that have sold to museums in the past.

“It’s the rarest thing that there is in terms of the art world,” said Simon in an interview with Artsy. “There are so few paintings by Leonardo and it’s extremely unlikely that another one will appear in our lifetimes.”

Don Thompson, author of the new book The Orange Balloon Dog: Bubbles, Turmoil and Avarice in the Contemporary Art Market, said that the bidders could offer anywhere from $25 million to $200 million.  “It’s all pretty speculative at this point,” he said.

He said that he wouldn’t be surprised if the painting ended up in a bidding war between the Louvre Abu Dhabi and the Qatar Museums—which would both love to have the treasure as a popular draw—and then it all depends on how flexible those museums would be in their ability to bid spontaneously. Other bids, he said, might come from “an oligarch in China or Russia.”

Some experts who spoke to Artsy suggest that the painting might fall short of price expectations because of doubts about the attribution. Frank Zöllner, an author of the most recent Leonardo da Vinci catalogue raisonné published in 2015, included the painting under the heading “Leonardo da Vinci and Workshop (?),” and called the attribution “controversial.”

In an email to Artsy, he said that he hasn’t changed his mind yet. “Christie’s wants to sell a once-in-a-lifetime trophy painting (in a contemporary art sale! with lots of trophy buyers around),” he said, but he still maintains his position that the technique used in the painting “corresponds more closely to the manner of a talented Leonardo pupil active in the 1520s than to the style of the master himself.”

Other Leonardo experts share concerns that the work is either only partially by him, and that others may have also had a hand in completing it. This is a common issue with Old Master paintings, because artists in the 15th through the 17th centuries typically worked in studios with assistants and pupils, who would be asked to contribute elements of the work.

Carmen Bambach, curator of Italian and Spanish drawings at the Metropolitan Museum of Art, in a 2012 article published in Apollo magazine, wrote that the work was “an important addition to the scholarship, but requires a more qualified description, for its severely damaged original painting surface exhibits large portions of recent integration.”

After having studied and followed the picture during its conservation treatment over six years from 2005 to 2011, and seeing it in the National Gallery, she concluded that “much of the original painting surface may be by Boltraffio, but with passages done by Leonardo himself,” referring to Giovanni Antonio Boltraffio, one of Leonardo’s pupils.

In its extensive provenance and history report, Christie’s dates the painting to around 1500. Luke Syson, in the catalogue to the National Gallery exhibition, speculated that it may have been made for the French royal family and that it was brought to England by Queen Henrietta Maria when she married King Charles I in 1625.

According to an inventory of the royal collection, it did belong to Charles I, and stayed in the royal collection until 1649, when he lost his head. The painting was sold at a Commonwealth Sale two years later to a mason named John Stone, who was required to return it to Charles II by an act of Parliament. It remained in the royal collection until the late 18th century, when it disappeared.

When it reappeared in about 1900 in the Cook Collection in the U.K., it had lost its link to Leonardo, and was instead attributed to Leonardo’s follower, Bernardino Luini, perhaps because it had been damaged and badly overpainted. It stayed in the Cook Collection until 1958, when Cook’s heirs sold it through Sotheby’s, this time attributed to Boltraffio. There, it fetched only £45 (about $60).

But then the work came to America, where it stayed until 2005, when it was sold to a consortium of art dealers, including Robert Simon and Alexander Parish, who thought it might be a Leonardo. Led by Simon, they conducted new research on the work, and presented it to the National Gallery in 2011 and at the Dallas Museum of Art in 2012 as a Leonardo. The painting was then back on the market, with an asking price of $200 million, Old Master dealer Bob Haboldt told the New York Times. The Dallas Museum reportedly made a bid of “tens of millions” a museum trustee, Deedie Rose told D Magazine, for the work.

But freeport magnate Yves Bouvier won it for reportedly $80 million in a private sale brokered by Sotheby’s in 2013. He turned around and sold it within months to Russian Billionaire Dmitry Rybolovlev that May, who paid $127.5 million. When Rybolovlev learned of the vast price difference, a pair of lawsuits ensued. Now Rybolovlev is selling.

Wintermute said that today, “the overwhelming, overwhelming, overwhelming point of view is that it is absolutely a work by Leonardo da Vinci, and that it is entirely a Leonardo da Vinci, and that it is the last work by Leonardo da Vinci still in private hands painted entirely by him.”

Among those who Christie’s lists as endorsing the painting are Larry Keith, director of collections and conservation at the National Gallery in London; David Ekserdjian, professor of art history at the University of Leicester; Nicholas Penny, former director of the National Gallery in London; Keith Christiansen, chair of European paintings at the Metropolitan Museum of Art; and Vincent Delieuvin, curator of 16th-century Italian painting at the Louvre.

Martin Kemp, Emeritus Research Professor in the History of Art at Oxford University and a leading expert on Leonardo da Vinci, said in an interview that the work is “convincingly by Leonardo,” and that the six years of restoration work and substantial technical analysis conducted by Dianne Dwyer Modestini only supported his certainty about the attribution. Kemp, Margaret Dalivalle, and Robert Simon co-authored Leonardo’s Salvator Mundi and the Collecting of Leonardo in the Stuart Courts, which Oxford University Press will publish in 2018.

Michael Daley, director of ArtWatch UK, an organization that aims to “counter partisan or propagandistic” art media, said that he’d like to see the final comprehensive analysis of the painting published before the work is sold as a definitive Leonardo.  “It used to be the case that if an art historian wanted to present an attribution, he or she would write a scholarly article and publish it in the scholarly press to invite debate,” he said. “Before they declare that it’s a Leonardo, it should be put on the record so that we can all look at the evidence and talk about it.”

Wintermute, Kemp, and Simon all said that they’re full convinced by the evidence, and that there’s no doubt in their minds about the attribution. It’s almost never possible to achieve universal consensus with Old Masters, in any case; there are usually dissenters.

“To say that it’s a controversial attribution is absolutely wrong,” said Kemp. “There have been a few doubters but for a new Leonardo to achieve such a general consensus is very remarkable.”