The Blue Grotto, the most famous tourist attraction on the island of Capri, was closed to visitors yesterday due to fears its crystal waters had been contaminated by raw sewage, possibly dumped by the Naples mafia.

La Grotta Azzurra, as it is known in Italian, is celebrated for the intense blue tones of the water and the mysterious, silvery light thrown off by underwater rocks in the cave. In the 1st century AD, the Emperor Tiberius liked to retire to the grotto to swim and cool off during the hottest days of summer.

But the cave's problems have been brewing since reports that two employees of a waste disposal firm at Castellamare di Stabia, near Naples, had dumped 5,000 litres of raw cesspool sewage in the grotto using hoses. The firm has the contract for the disposal of waste from homes and hotels on the island to sewage treatment plants in the area.

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The two men are alleged to have dumped the sewage to avoid paying charges levied at the treatment plants. They were arrested and are currently under house arrest. An inquiry into the polluting of the famous cavern started last week.

But on Tuesday afternoon, the longer-term implications of the affair became apparent after boatmen ferrying tourists around the inside of the cavern discovered evil-smelling cream-coloured scum floating in it. One of the boatmen became nauseous after inhaling the fumes, and some of the tourists said they felt sick.

Health experts were dispatched to test the waters to identify the noxious substances, and police said the grotto would not be reopened until their work was completed.

The Naples region, which includes Capri, has for years been beset by problems connected with rubbish disposal, largely because of the success of the Camorra, the local equivalent of the Sicilian Mafia, in infiltrating the industry. Last year, in the early weeks of his new government, Prime Minister Silvio Berlusconi staked his credibility on securing a speedy end to a crisis which had seen the streets of the city, Italy's fourth largest, stacked up to head height with huge piles of uncollected domestic refuse.

Yesterday his Environment Minister, Stefania Prestigiacomo, claimed that the Camorra might also be behind the poisoning of the grotto.

Ms Prestigiacomo said the overall environmental situation on the island was "disturbing". "Structural problems, lack of infrastructure, a sewage system that is incomplete, with not all homes connected to it: this is a situation which encourages the infiltration of criminal gangs," she claimed.

"From what I have learned, the conditions [on the island] could lend themselves to being manipulated by illegal interests. The suspicion is in the air. I hope that the investigators will get to the bottom of it and make their findings known as soon as possible."

But a public prosecutor in Naples was quick to deny the likelihood of organised crime being involved. "As things stand now there are no elements to lead us to suppose that the Camorra is involved in these latest episodes on Capri," Giovandomenico Lepore, the public prosecutor of the city, said. "It seems to me a hasty and premature judgment to raise the bogey of the Camorra. To date there has been no evidence that organised crime has a presence on the island."

The Blue Grotto is entered by a narrow aperture in the high rocky wall some two metres wide by two metres high, requiring tourists, arriving two or three to a rowing boat, to enter lying down while the boatman hauls his craft inside with a chain.

The uniquely vivid blue of the light inside the cavern is caused by daylight entering through an underwater opening beneath the above-water entrance. The light is filtered by the water which absorbs the red tones, leaving the light in the cave a startlingly blue hue.