It was anything but a false alarm.

When police reacted to a warning call at a bank in Antwerp’s famed diamond center, they found the vault door still secured. It was only after they forced their way inside that they realized the sheer audacity of the robbery that had taken place — some 30 empty deposit boxes, a hole in the floor, a tunnel to the sewage system.

It’s a mystery worthy of a Hollywood heist movie.

Though police have so far been unable to quantify what the robber or robbers made off with, clients were lining up by the dozen on Monday to find out if their boxes had been emptied.

“We are investigating the size of the robbery and we cannot provide further information,” the prosecutor’s office said in a statement.

What’s clear is that the heist was as daring as it was dangerous. First, a tunnel had to be dug from a home several hundred meters away into the sewage system. Then, a move had to be made through the claustrophobic sewers, measuring less than a meter-wide toward the bank that could at any time get filled with water or noxious vapors.

And then? They had to dig a second tunnel to get up into the vault of the BNP Paribas branch.

After that, it was a case of getting as much as possible. It was a weekend so the perpetrator or perpetrators had perhaps a bit more time.

According to the prosecutor, the bank’s security firm called about a burglary in the vault at 1:34 p.m.

When police arrived, they saw the vault was “still locked but the alarm was on.” Once opened, they found the “hole in the floor.”

With the help of the firefighting department they checked the sewage system in the neighborhood until they found the second tunnel.

“At the moment no suspects could be apprehended,” the prosecutor said in a statement.

Customers at the bank could not immediately check whether their deposit boxes were involved.

The heist had the hallmarks of the Nice robbery in France in 1976 when 339 safe deposit boxes were cleaned out at the Societe Generale bank by the “sewer gang” on the summer weekend.

(AP)