This story is a collaboration with Biography.com.

The Nazis’ systematic theft of art and other valuables during the Second World War, while less horrifying than the human cost of their quest for global domination, continues to impact our cultural landscape today. Even after the passage of eight decades, art looted by Hitler’s forces is still being painstakingly tracked down and recovered.

Making a full accounting of the artworks that the Nazis plundered would be a daunting task, with the National Archives and Records Administration estimating that “upwards of 20% of the art of Europe was looted by the Nazis.” But of all the stolen works, the most opulent, the most sprawling, and likely the most valuable (estimated at around $504.1 million) is the Amber Room.

Far from a single portable artwork, the Amber Room was both museum piece and gallery, a full room measuring nearly 600 square feet, replete with carvings, gemstones, gold, and amber panels weighing a total of roughly 13,000 pounds. That this massive masterpiece, once dubbed the Eighth Wonder of the World, was somehow relocated at least twice (once before the Nazi theft) is remarkable enough. That it has managed to disappear completely in the eighty years since is one of the greatest mysteries of the art world.

The Amber Room originated in the city of Berlin, in what was then the country of Prussia. Conceived in 1701 for the Charlottenburg Palace, the room was intended to be a space so opulent that it could rival France’s famous Palace of Versailles. It was ultimately completed in 1712 and installed instead into the Berlin Palace, roughly five miles away, as part of an expansion that King Frederick I ordered the sculptor Andreas Schlüter to carry out.

The next year, in 1713, King Frederick I died, and his successor, King Frederick William I, saw this stunning room not as an opportunity to flaunt, but rather to further strengthen alliances. That’s why, in 1716, he gifted the entire completed room to another country, the Russian Empire, in order to secure his alliance with Tsar Peter I.

As SmithsonianMagazine notes, the room was disassembled and shipped to Russia, where it was first displayed in the Winter House in St. Petersburg. In 1755, Czarina Elizabeth had it relocated to the Imperial residence of Ца́рское Село́, which translates to Czar’s Village (the town’s current name is Pushkin, named for the poet Alexander Pushkin in 1937), and installed in the Catherine Palace. It was there that the room was augmented and expanded into its fullest form, and there that it stayed until the Nazi scourge broke through the Russian border.

Anticipating the German invasion known as Operation Barbarossa, the Soviet Union took measures to protect their cultural artifacts from Nazi plunder. In Leningrad, Hermitage Museum director Josef Orbeli and his team managed to shepherd hundreds of thousands of artworks out of the Nazis’ grasp (and later testified before the Nuremberg Tribunal about the Nazis’ wanton destruction of cultural artifacts). But Anatoly Kuchumov and the curators at the Catherine Palace found themselves unable to extricate the intricate amber panels that had once been disassembled and transported to them two centuries earlier. The lustrous amber had become too brittle over the years, so rather than risk destroying it during the hurried transport, they instead tried to hide it behind a makeshift wallpaper which, according to a translation of a Russian-language source, was made of “paper, gauze, and cotton wool.”

Unfortunately, the Nazis could not be deceived by the hectically-placed wallpaper. They knew well what awaited them within the Catherine Palace, in no small part because the Amber Room was featured in the infamous Kümmel Report. In 1940, Joseph Goebbels commissioned art historian and Nazi Party member Otto Kümmel to compile a list of artwork of allegedly “Aryan origin” that would be taken by the Third Reich. Kümmel’s list would total more than 300 pages, incorporating any artwork that had arguably ever been in German possession over the preceding three centuries. Thus the Amber Room’s origins in Berlin made it a Nazi target for “reclamation.”

In October 1941, after having been disassembled by the Nazis during a painstaking 36-hour process, the Amber Room was relocated once again, this time to Königsberg Castle in the city of Königsberg, which was then headquarters to the Wehrmacht’sWehrkreis I. But what became of the Amber Room after that is unclear. By the time the war was over, the Eighth Wonder of the World had vanished without a trace.

After the war, Königsberg fell into the possession of the Soviet Union, who renamed the city to Kaliningrad (today, the Kaliningrad Oblast is still a part of modern-day Russia, though it is sandwiched between Poland and Lithuania and shares no borders with mainland Russia). But though the city now belonged to the Soviets, the marvelous room that had been stolen from them was still nowhere to be found.

Rumors swirled that pieces of the room had been loaded onto the Nazi Kriegsmarine vessel MV Wilhelm Gustloff, which had been used to evacuate civilians and military personnel from East Prussia as the Soviet Red Army beat back the Nazis. If so, then on January 30, 1945, when the Soviet submarine S-13 torpedoed the Wilhelm Gustloff and sank it, the Amber Room was lost along with the lives of what Time estimated were 9,343 people, the majority of whom were war refugees and children.

Others still clung to the idea that the Amber Room had somehow survived. Perhaps it had been successfully smuggled out by someone, or perhaps it never even left Russia at all; one theory suggested that Stalin had the Amber Room replaced with a decoy before the Nazis came, squirreling the real one away for safekeeping. But if the “decoy room” theory were true, it remains unclear why the original room never re-emerged after the war. Instead, the USSR undertook an Amber Room replica project in 1979, and while some fringe theories claimed that the recreation was actually the real one brought out of hiding, it’s hard to explain why it would take nearly 24 years just to reinstall the allegedly-already-existing Amber Room in the Catherine Palace.

The “smuggled out” theory gained some credence in 1997, when one of the four Florentine mosaics that had once adorned the Amber Room resurfaced in Germany, but since then, no other remnants have emerged. Adding to the intrigue are the bizarre fates that befell several figures who actively searched for the Amber Room. General Yuri Gusev died in a car accident shortly after allegedly serving as a source for a journalist investigating the Amber Room, while German soldier and outspoken Amber Room hunter Georg Stein was found disemboweled in a Bavarian forest.

In their 2004 book The Amber Room: The Untold Story of the Greatest Hoax of the Twentieth Century by Guardian, journalists Catherine Scott-Clark and Adrian Levy exhaustively researched Soviet records and discovered something astounding. Key Soviet figures, including Catherine Palace head curator Anatoly Kuchumov, knew as far back as 1946 that “in a cataclysmic error of judgment or an orgy of vengeful violence, undisciplined Soviet soldiers had allegedly torched the most valuable missing treasure in the world.”

The final assault of the Battle of Königsberg was an incredibly destructive four-day clash between the Nazis and the Red Army, and it left a devastating mark on the city. As Isabel Denny describes in The Fall of Hitler’s Fortress City: The Battle for Königsberg, 1945, “The city fell in ruins and burned.” That burning included Königsberg Castle, which was repeatedly shelled by the Red Army. If the fragile resin of the Amber Room had managed to withstand the RAF bombing that occurred the previous year, this shelling would certainly have finished the job.

If so, then all the suspect behavior that fueled the Amber Room survival conspiracy theories over the years, including supposed “eyewitness accounts” of Nazis smuggling the room out of Königsberg, or of Soviet general secretary Leonid Brezhnev ordering the destruction of Königsberg Castle in 1968, weren’t actions taken to obscure the path to a hidden treasure. Rather, these rumors were purposefully perpetuated to hide the embarrassing secret that the Soviet Union had accidentally destroyed one of their country’s most treasured and valuable artworks in the fog of war.