At least £22million in cash and jewellery has been stolen after four men tunnelled into the vaults of a Paris bank and emptied 200 safes.
The heist - already being described as France's 'crime of the century' - saw the gang start digging in a neighbouring cellar, before using pickaxes and a even a flame thrower to break through 31in thick walls to the vaults.
Once inside the Credit Lyonnais branch, which was closed for refurbishment, they tied up a guard and warned him he would be shot if he did not stay still.
They then spent 'a leisurely time' breaking open and emptying the high-security private safe deposit boxes, police said.
The gang, nicknamed The Termites, is thought to have spent several days weakening the walls before they struck.
A police officer added: 'The whole thing was clearly meticulously planned - the criminals were ice cool and determined.'
The robbers entered the bank, in the upmarket Avenue de l'Opera, at around 10pm on Saturday and left at 7am on Sunday. After filling bags with cash, jewels and valuables, they started a fire which set off sprinklers to flood the basement and destroy evidence.
A spokesman for the Paris Criminal Brigade said the heist bore the hallmarks of previous robberies of smaller banks around the French capital in which walls were broken down during periods of refurbishment.
A spokesman for Credit Lyonnais said the branch, near the Louvre Museum, had been closed for weeks while being modernised. 'Unfortunately we weren't able to remove the safety deposit boxes during this period,' he added.
The crime has echoes of a £6million robbery in Nice in 1976 when self-styled master criminal Albert Spaggiari spent two months drilling a 25ft tunnel into a bank.
His gang spent four days in the bank over a holiday weekend and left a message behind which read: 'Without hatred, without violence, without weapons.'
The branch is situated at the bottom of an office block which was completely deserted at the weekend. There was only the one private security guard on duty.
The Paris Criminal Brigade is investigating the latest heist. A spokesman said the fire had ‘undoubtedly damaged the crime scene, making the compilation of evidence difficult.’
He said no exact figure could be put on what they got away with, as it was not known exactly what was in the boxes.
However, he estimated that the cash and valuables would be worth ‘many millions'.