30 Historic Treasures We 100% Know Exist But Haven’t Tracked Them Down

The tomb of the First Qin Emperor of China exists and we know exactly where it is. The only part of the site that's been extensively excavated for artifacts is the famous Terracotta Army.

The main tomb itself is still completely sealed and no attempts have been made to open it since legend states that it was filled not only with mountains of treasure inside, but also mechanically operated rivers of flowing mercury made to resemble the major rivers and bodies of water in China and booby traps to protect the massive subterranean complex, which has been measured to be around 1/4th the size of the Forbidden City using ground-penetrating radar and other non-invasive techniques.

Apparently archeologists and scientists have found disturbingly high levels of mercury in the ground around the tomb giving some air of legitimacy to the legends which also raises concerns about the legend of booby traps. There's also concern that we do not currently have the technology and knowledge to protect artifacts we'd excavate from outside air and contamination, just like how earlier excavations of the Terracotta Army led to their original colorful paint being destroyed shortly after being unearthed.

EDIT: Since this comment got so popular, I'll add on the "Heirloom Seal of the Realm" also known as the "Imperial Seal of China". Physical possession of the object basically legitimized a claim to the Mandate of Heaven and thus rule over China. It was created in 221 BC, passing along through various emperors and factions until sometime between around 1500-1000 years ago. There's several theories on how, when, and where it was lost but we'll probably never get an answer unless some farmer happens to stumble on a fairly big square hunk of intricately carved jade in some field and realizes what it is.

A large part of Pompei. What they have excavated so far takes an hour to walk through, but a huge part of it remains hidden. This is because they've found that being discovered is causing more damage to the remains than 2,000 years of being hidden beneath the pumice and they're waiting for new technology to preserve it.

The missing Faberge eggs. There are about seven still missing, and only an old black and white photo remains of some of them, along with their descriptions.

Several have mentioned Genghis Khan’s tomb, but I’m more interested in his Black Spirit Banner which survived up until the 1960’s or so before disappearing under Soviet control of Mongolia.

The Black Spirit banner was made from black horse hair and carried by the Khan into battle and displayed during times of war. Similar to the mythos surrounding the Spear of Destiny, it was said the warriors of Mongolia would always be victorious in battle so long as they had the banner. After the Soviet Union took de facto control over the country in the 1950’s, the banner was held in a government office until it went missing sometime in the 70’s.

The book “Genghis Khan and the Making of the Modern World” by Jack Weatherford is a great read if you’re interested in learning more about the history of the Khans and their impact on our world today.

"The Amber Room (Russian: Янтарная комната, tr. Yantarnaya Komnata, German: Bernsteinzimmer) was a chamber decorated in amber panels backed with gold leaf and mirrors, located in the Catherine Palace of Tsarskoye Selo near Saint Petersburg" 

People are still looking, mostly in Poland

The enormous golden menorah from the 2nd temple.

The Romans took it after sacking Jerusalem. Some people think it’s being hidden by the Vatican, but I think more likely it was melted down and will never be found.

Tomb of Alexander the Great. Cleopatra and Marc Antony. Funny thing is, there are all probably buried in Alexandria beneath the modern city.

Not that it isn't found, but the Forbidden Vault (Vault B) in the Padmanabhaswamy Temple in India remains unopened and is thought to contain treasure to a wealth of US$ one trillion. Other vaults opened there were found to contain 18' gold chains, a solid gold sheaf weighing 500kg and a three-and-a-half feet tall solid pure golden idol of Mahavishnu, studded with hundreds of diamonds and rubies and other precious stones.

The reason they don't open Vault B? It's said to be cursed. 

I went to college beside that museum in Boston that got robbed. They never found the thieves or the paintings. The famous Gardner museum theft. No one knows what happened to the paintings still.

Almost all Homo Erectus Pekinensis remains. They were packed up during WWII with the intention of shipping them from China to the US for safekeeping, but the ship supposedly carrying the fossils was attacked and ran aground. Photographs and drawings of the originals still exist but the actual fossils were never found.

This one really isn’t talked about as much as it should. In 1996, billionaire couple Marvin and Kaye Lichtman who collected tons upon tons of priceless items art, chess sets, Faberge eggs, etc were murdered by a glass installer that they hired. Before burning the house down, the glass installer made off with as much of the couple’s riches as he could haul away. Shortly after he tried to flee to avoid prosecution and before he was caught, 3 men involved with the salvage operation at the scene were arrested for peddling items that were salvaged and they stole. Most of the lichtman’s collection including the items stolen by the glass installer who murdered them have never been found.

King Johns Crown Jewels. Lost while crossing the Wash on the east coast of England. Over 800 years ago

The shipwreck of Flor de la Mar. Sunk with the equivalent of $2 billion in todays money.

The horde of Aztec gold stolen and lost by Hernan Cortez and the Spanish conquistadors during the night of sorrows.

While trying to escape from Tenochitlan in the night with the gold, the conquistadors were set upon by the Aztecs. During the fighting and confusion it's thought the gold fell into lake Texcoco. Hernan Cortez escaped with only a small number of his soldiers and the gold has never been found. 

$190 million worth of bitcoin on a hard drive in a particular landfill. 

Portrait of a Young Man by Raphael. Missing since the end of ww2, all signs point to it having been sold for traveling money by Hans Frank.

No one mentioned the Ark of the Covenant.

The Honjo Masamune samurai sword.

Irish Crown Jewels.

Tomb of Genghis Khan

Olavsskrinet; The sarcophagus of Olav Haraldsson.

Remember the christian viking from Vikings: Valhalla? Yeah, he was real. After his death, he became a saint and a bridge between old Norse faith and Christianity.
He was eventually laid to rest in a jewel adorned sarcophagus in the Nidaros cathedral. It was Northern Europes biggest destination for pilgrims until the reformation.

When Norwegians broke with the catholic church, Olavsskrinet - maybe with Olav still in it - disappeared. Buried under the cathedral? Sent to Denmark? Nobody knows.