A group of thieves, including a security guard in on the job, used a wheelbarrow and ladder to steal one of the world's largest gold coins from a German museum in a daring late-night heist, prosecutors allege.

Key points: The ultra-pure gold coin was stolen from Berlin's Bode Museum in March 2017

The ultra-pure gold coin was stolen from Berlin's Bode Museum in March 2017 Authorities believe the alleged thieves extracted the coin from a bullet-proof glass case

Authorities believe the alleged thieves extracted the coin from a bullet-proof glass case The coin has a face value of $1 million, but is thought to be worth four times as much

Four men are on trial over the theft of the Canadian "Big Maple Leaf", a 100-kilogram gold coin once recognised as the biggest in the world.

Authorities believe the coin, which is the size of a manhole cover, has been melted down since it was stolen from Berlin's Bode Museum in March 2017.

As the trial begins in Berlin, prosecutors told the court three of the suspects broke into the museum through an upstairs window and used a ladder, wheelbarrow and rope to extract the "Big Maple Leaf" from a bullet-proof glass.

The fourth suspect was a museum guard accused of helping them.

The suspects — three of whom are related — hid their faces behind magazines as they entered court, charged with serious theft.

German media reported the men could face up to three years and 10 months jail if convicted.

The coin, more than half a metre in diameter, 3 centimetres thick and made from ultra-pure gold, is one of just six produced by the Royal Canadian Mint in 2007 and was loaned to the museum by a private owner.

It has a face value of $1 million, but is thought to be worth four times as much.

It was recognized by the Guinness Book of World Records as the largest gold coin in the world at the time it was made, although Australia has since minted one even bigger.

Reuters/ABC