One of the world's largest gold coins is a 221-pound behemoth from Canada called the Big Maple Leaf. Until this week, the 21-inch wide, inch-thick coin had been housed in Berlin's Bode Museum, but it was stolen on March 27, 2017 — and police have no idea how the thieves pulled it off.

The face value of the coin, which has the head of Queen Elizabeth II on one side and a maple leaf on the other, is 1 million Canadian dollars, or about $750,000, but by gold content alone, it's worth as much as $4.5 million.

Police say burglars apparently used a ladder and broke in through a window above some railway tracks around 3:30 a.m. — a time of night when the trains stop running. From there, they had to smash through the bulletproof glass surrounding the coin, and lug the hefty item through the museum, up a flight of stairs and out the window, the New York Times reports.

Police are asking the public for any information they may have. In the meantime, the theft got us thinking about past robberies where thieves got away with priceless objects or large amounts of cash. While it's too early to say whether the Berlin coin robbery will go down in history like the following 10 unsolved heists, it definitely fits the bill as an interesting crime caper.

1. The Isabella Stewart Gardner Museum robbery in Boston

Empty frames hang at the Isabella Stewart Gardner Museum as placeholders for when the stolen works of art are returned. Empty frames hang at the Isabella Stewart Gardner Museum as placeholders for when the stolen works of art are returned. (Photo: Federal Bureau of Investigation/Wikimedia Commons)

On March 18, 1990, two men disguised as police officers walked into the Isabella Stewart Gardner Museum in Boston and told the security guard they were responding to a call. The guard let them enter, but once inside, they handcuffed that guard and a second one, and locked them in the basement.

They got away with 13 extremely valuable pieces of art worth $500 million, including Rembrandt’s "Storm on the Sea of Galilee" (1633), "A Lady and Gentleman in Black" (1633) and a self portrait from 1634; Vermeer’s "The Concert" (1658–1660); Govaert Flinck’s "Landscape with an Obelisk" (1638); five Edgar Degas’ impressionist works; and Edouard Manet’s "Chez Tortoni" (1878–1880).

To this day, no one knows who the robbers were or where they hid the goods from the largest theft of private property in history. Empty frames hang in the museum as placeholders for when the stolen works are returned. The Gardner Museum is offering a $5 million reward for information leading to the recovery of these works in good condition.

2. The Tucker Cross theft

In 1955, a Bermudian man named Teddy Tucker was scuba diving in the wreckage of the San Pedro, a Spanish ship that sunk near the Florida Keys during a hurricane in 1594, and he found this 22-carat gold-and-emerald cross. He brought it home and sold it to the government of Bermuda, and it was displayed in a museum on the island (that he and his wife owned and ran) for several years.

However, in 1975, just before an official visit by Queen Elizabeth II, the cross was stolen and replaced with a cheap replica. Authorities don't know who stole the cross — which was considered to be the most valuable object ever found in a shipwreck — or where it may be now.

3. The Antwerp diamond heist

Antwerp World Diamond Centre logo The Antwerp World Diamond Centre in Belgium was the site of a $100 million diamond heist in 2003. (Photo: Julien Warnand/AFP/Getty Images)

The Antwerp World Diamond Centre (AWDC) in Belgium is the diamond-exchange capital of the world, and in February 2003, it was the site of a $100 million diamond heist.

As U.S. News and World Report says:

A group of Italian thieves known as the "The School of Turin" broke into the underground vault of the Antwerp Diamond Center, then protected by infrared heat detectors, sophisticated locks [with 100 million possible combinations], and eight other layers of security. Despite this, the gang successfully looted 123 of the vault's 160 safes without setting off any alarms or leaving behind any signs of forced entry — security did not notice until the following day.

An Italian man named Leonardo Notarbartolo (a career thief) was convicted of being the ringleader and has since been paroled. He had rented an office in the AWDC shortly before the robbery and used its location to gain access to the bank vault. But he never gave away his accomplices or the location of the diamonds.

4. The Plymouth mail truck robbery

Many people believes the three accused gunmen were not convicted because of the sudden disappearance of the fourth, Thomas Richards, who was slated to testify against the rest of the group. Many people believe the three accused gunmen were not convicted because of the sudden disappearance of the fourth, Thomas Richards (bottom right), who was slated to testify against the rest of the group. (Photo: Smithsonian National Postal Museum)

In August 1962, a team of criminals dressed as police officers and armed with guns ambushed a mail truck traveling from Plymouth, Massachusetts, to the Federal Reserve Bank of Boston. Using an elaborate scheme involving fake highway workers and traffic detours, the men got away with $1.5 million in cash — all in bills smaller than $20, and only some of it recorded — in what was, at the time, the largest cash heist in history.

The postal workers were blindfolded, bound and gagged, and put in the back of the truck. One of the men (authorities believe there were six of them) got in the driver's seat and drove for a while before abandoning the truck with the mailmen still inside.

As GateHouse Media's Wicked Local reports:

[U.S. Postal Service inspectors] worked hand in hand with the State Police, FBI and other law enforcement agencies, and managed to uncover evidence from the homes of two of the criminals, as well as eyewitness testimony. ... A grand jury indicted three suspects, but the linchpin of the case, Tommy Richards, who was slated to testify against the others, disappeared mysteriously, never to be seen again.

The remaining defendants were found not guilty, and the money was never recovered.

5. D.B. Cooper and a stolen plane

D.B. Cooper's plane ticket On Nov. 24, 1971, a man calling himself Dan Cooper used cash to buy a one-way ticket on Northwest Orient Airlines from Portland, Oregon, to Seattle, Washington. (Photo: Federal Bureau of Investigation/flickr)

In November 1971, a cunning air pirate known as D.B. Cooper skyjacked Northwest Orient Airlines Flight 305 headed from Portland, Oregon, to Seattle-Tacoma International Airport. About 30 minutes after takeoff, Cooper told a flight attendant he had explosive devices and demanded $200,000, four parachutes and a refueling truck upon landing at Sea-Tac.

Indeed, once the plane landed, Cooper's requests were met, and he released the passengers before taking off with a pilot and a handful of crew members for his desired destination of Mexico City. However, Cooper didn't intend to complete the journey. He strapped on a parachute and, from 10,000 feet in the air, jumped out of the plane into the night 30 minutes after taking off from Sea-Tac.

To this day, we do not know who D.B. Cooper was, and the FBI has processed thousands of suspects in the case of America's only unsolved skyjacking.

Arrests, prosecutions ... but no loot

In the following five robberies, arrests were made and suspects were prosecuted, but the stolen goods were never recovered. In some cases, authorities believe the cash or jewels may never be recovered.

6. Banco Central robbery in Fortaleza, Brazil

A tunnel that thieves dug to get from their office into the Banco Central in Fortaleza, Brazil A tunnel that thieves dug from their office into the Banco Central in Fortaleza, Brazil. (Photo: Turtle Mag/Facebook)

The Guinness Book of World Records awarded this heist the title of "greatest robbery of a bank," and the plot sounds like something straight out of a movie.

In 2005, a group of men rented a property and set up shop posing as a landscape company a few blocks from the Banco Central in Fortaleza, Brazil. They spent three months digging a tunnel about 256 feet long and 13 feet below street level from their office to directly below the bank.

Over the course of a weekend in August, they used the tunnel to get into the bank and managed to avoid or disable all the bank's censors, thanks to a tip from a bank employee. From there, they broke through nearly 4 feet of steel-reinforced concrete to enter the vault and stole five containers weighing more than 7,000 pounds and holding about $70 million worth of reals (Brazilian currency).

Bank employees didn't know anything had happened until they arrived at work Monday morning. And by then, the robbers had already fled the area. However, they made two mistakes that led to their demise. As OZY reports:

Outside, police would later find a large amount of white powder — chalk the robbers had used to cover their fingerprints. And they nearly succeeded, except for one print, their first slip. The second mistake? A member of the gang bought 10 cars at once the next day, paying cash and raising red flags in this poor region of Brazil. Improbably, the police managed to catch up with the trailer carrying those cars in another state, and inside three of the vehicles were bundles of 50 real bills.

Three dozen people were accused of participating in the heist; 26 ended up in jail for various crimes, and a few of them escaped. But only about $8 million of the total amount was ever recovered, making this the biggest robbery in the history of Brazil.

7. The Great Train Robbery in England

This is the view near Ledburn, Buckinghamshire, over the West Coast Main Line, toward Sears Crossing where robbers took control of a mail train during the Great Train Robbery in 1963. This is the view near Ledburn, Buckinghamshire, over the West Coast Main Line, toward Sears Crossing where robbers took control of a mail train during the Great Train Robbery in 1963. (Photo: Sealman/Wikimedia Commons

On Aug. 8, 1963, a train going from Glasgow to London was ambushed on the Bridego Railway Bridge in Buckinghamshire by a group of 15 robbers who rigged the track signals to stop the train in a remote location.

The robbers didn't have guns, but they did beat up the train driver before running away with over £2.6 million (equivalent to $61 million U.S. today). They fled to a hideout, which police would later find and collect evidence from to prosecute most of the gang. However, the money was never recovered.

Ringleaders were sentenced to 30 years in prison, including Ronald Arthur "Ronnie" Biggs, who later escaped, and Bruce "Napoleon" Reynolds, who went on to work as a consultant for a movie ("Buster," released in 1988) and published "The Autobiography of a Thief: The Man Behind The Great Train Robbery" in 1995.

8. Dunbar robbery in Los Angeles

Dunbar armored vehicle In 1997, a group of men robbed the Dunbar armored truck depot in Los Angeles and got away with nearly $19 million in cash. (Photo: Tupungato/Shutterstock)

In September 1997, at least six men stole $18.9 million in cash from the Dunbar armored truck depot in Los Angeles. Their evening began at a house party in Long Beach, where they went to establish an alibi. But they sneaked out shortly thereafter, changed into black clothing, and drove to the depot, entering through a side door shortly after midnight. They tied up the few employees who were working and forced them to lie face down on the floor.

As the L.A. Times reports:

The armed robbers advanced on the vault area... and, using bolt cutters, broke the padlocks on metal cages containing the depot's cash. Most of the currency consisted of $20 bills, destined for drop-offs at automated teller machines throughout the Los Angeles area. The robbers tossed the money into metal carts, which they wheeled to the building's loading dock and dumped into a U-Haul truck that one of them had rented for the robbery. Before departing, they smashed all of the security video cameras inside the depot and seized the videotapes.

The U-Haul was their undoing. Somehow, a plastic taillight lens fell off at the scene, which the FBI later matched to the rented U-Haul. The mastermind, Allen Pace III, was a former security officer for Dunbar who was very familiar with the security process, prosecutors said. He was convicted along with the rest of the group — four of whom pleaded guilty. While authorities recovered about $5 million of the cash in the form of homes, cars and other valuables, the remaining amount — more than $10 million — was never recovered.

9. Brink's-Mat robbery in Britain

Brinks truck The Brink's-Mat robbery was the largest gold heist in British history. (Photo: GeorgHH/Wikimedia Commons)

In the morning hours of Nov. 26, 1983, six men wearing balaclavas entered a warehouse at London's Heathrow Airport belonging to security company Brink’s-Mat. The warehouse vault contained more than $3 million in cash, which the robbers knew because they had help from the inside. What they didn't know was that the vault also contained more than three tons (7,000 bars) of gold bullion.

The armed men tied up the guards and poured gasoline on them, threatening to light a match if they didn't offer up the keys and the codes to the vault. The thieves loaded the gold into a van and drove off, but they weren't free for very long. The inside man, Anthony Black, was implicated fairly quickly and squealed on his comrades. Another not-so-smart robber, Micky McAvoy, reportedly used his cut to pay cash for a house and bought two security dogs, which he named Brinks and Mat, to guard the property. He and Black’s brother-in-law, Brian Robinson, were sentenced to 25 years in prison.

You can read more about the fate of the robbers in this Metro story, but police never recovered most of the gold.

10. The Harry Winston heist

The Harry Winston jewelry store on Avenue Montaigne in Paris was robbed by four men dressed as women in 2008. The Harry Winston jewelry store on Avenue Montaigne in Paris was robbed by four men dressed as women in 2008. (Photo: Ralf.treinen/Wikimedia Commons

The posh Harry Winston jewelry store in Paris was the scene of a 2008 smash-and-grab robbery in which four men dressed as women stormed into the store, pushed employees and customers into a corner at gun-point, stole almost every piece of jewelry on display and emptied two storage cases in the back. They made a fast getaway with more than $100 million in merchandise, making it the largest jewel robbery ever in France and one of the largest in the world.

The thieves appeared to have inside knowledge of the store, The Guardian reports, because they knew the location of supposedly top-secret storage boxes and referred to staff by their first names. Eight men were arrested in what the French media dubbed the "steal of the century." The man believed to be the mastermind, Douadi Yahiaoui, was sentenced to 15 years in prison, the BBC reports, while others received as little as nine months in jail.

According to the BBC, police found $19 million worth of jewelry from the heist stuffed in a drain in the Parisian suburb of Seine-Saint-Denis, but much of the loot has never been recovered.

Angela Nelson ( @bostonangela ) is an exhausted mom of two young daughters and two old cats, and a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide.