In a brazen bank heist worthy of a Hollywood movie starring George Clooney and Brad Pitt, thieves broke into a bank vault holding deposit boxes full of jewels through sewage pipes under the Belgian city of Antwerp.

The authorities were initially left baffled upon arrival to the BNP Paribas bank on Sunday in response to the security alarm going off, with the vault doors secured and no signs of criminal activity.

They only grasped the seriousness of the heist after entering the vault and seeing some 30 empty deposit boxes, a hole in the floor, and a tunnel to the sewage system.

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Police are unable to quantify the exact damage caused by the thieves, with many clients of the bank checking whether their possessions were stolen. “We are investigating the size of the robbery and we cannot provide further information,” the prosecutor’s office said in a statement.

The bank is located in the famed largest diamond district in the world that claims of annual turnover of $54 billion, according to the Guardian.

Detectives have so far determined that the complex robbery involved digging a tunnel from a home several hundred meters away into the sewage system.

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The bank robbers then had to make their way through tight sewers – less than a meter wide – towards the bank, while risking the prospect of being flushed with water or harmful vapors. Upon arriving at the bank, the criminals had to dig another tunnel to get into the vault of the bank.

“A tunnel to the sewer system has been found. We first had measurements carried out on gas to see if it was safe to descend into it. In the sewerage we then found a second tunnel that led to a building in the Nerviërsstraat,” Willem Migom, a spokesman for Antwerp police, told the Guardian.

The investigators speculate that the robbers may had more time to empty the vault as it was a weekend and the bank was closed, though the theory remains uncorroborated.

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A prosecutor said the bank called the police about the burglary around 1:34pm on Sunday. They say the vault was “still locked but the alarm was on.” Only after entering the vault they saw the “hole in the floor.”

No suspects have been arrested in relation to the crime, the prosecutor said.

The Associated Press contributed to this report.