The search for lucre was filthy indeed for six Italians jailed this week over a bid to burrow up from the sewers of Naples into luxury shops and make off with jewels worth hundreds of thousands of euros.

Three were caught in the act last November and dragged out of the underground tunnels in handcuffs by agents who specialise in underwater operations, Carabinieri police said. The same three, together with three accomplices, were detained on Monday and have been jailed pending trial.

The band targeted boutiques including jeweller Bulgari, and had expected to net 300,000 euros ($330,450) each from a foiled raid on upmarket jeweller and clothier Bruno & Pisano.

They slipped into the sewers through manholes in the city's Spanish Quarter, traversed the honeycomb of tunnels below ground and dug judicious exit points under the shops.

Police found the holes and intervened to foil six robberies in all, police captain Michele Centola said, adding that five of the six thieves had already served jail time for the same kind of crime.

"It's a deep-rooted problem here," Centola sighed.

The investigation is ongoing.