Porto Ercole, Italy ? Stone fortresses and watchtowers that centuries ago stood guard against marauding pirates loom above pristine waters threatened by a modern peril: fuel trapped within the capsized Costa Concordia luxury liner.

A half-million gallons of heavy fuel oil is in danger of leaking out and polluting some of the Mediterranean’s most unspoiled sea, where dolphins chase playfully after sailboats and fishermen’s catches are so prized that wholesalers come from across Italy to scoop up cod, lobster, scampi, swordfish and other delicacies.

“Even the Caribbean has nothing on us,” said Francesco Arpino, a scuba instructor in the chic port of Porto Ercole, noting how the sleek granite sea bottom helps keep visibility crystal clear even 135 feet down.

Divers in these transparent waters marvel at an underwater world of sea horses and red coral, while on the surface sperm whales cut through the sea.

But worry is clouding this paradise, which includes a stretch of Tuscan coastline that has been the holiday haunt of soccer and screen stars, politicians and European royals.

Rough seas hindering divers’ search for bodies in the Concordia’s submerged section have also delayed the start of a pumping operation expected to last weeks to remove the fuel from the ship. Floating barriers aimed at containing any spillage now surround the vessel.

According to the Dutch salvage firm Smit, which has been contracted to remove the fuel, there are about a half million gallons of heavy fuel oil on board, as well as some 200 tons of diesel oil and smaller amounts of lubricants and other environmentally hazardous materials.

The ship lies dangerously close to a drop-off point on the sea bottom. Should strong waves nudge the vessel from its precarious perch, it could plunge some 90 feet, further complicating the pumping operation and possibly rupturing fuel tanks. Italy’s environment minister has warned that if the tanks break, the thick black fuel would block sunlight vital for marine life in the seabed.

A week after the Concordia struck a reef off the island of Giglio, flipping on its side, its crippled 114,000-ton hull rests on seabed rich with an underwater prairie of sea grass vital to the ecosystem. Environmentalists warn the sheer weight of the wreckage has likely already damaged a variety of marine life, including endangered sea sponges, and crustaceans and mollusks, even before a drop of fuel leaks.

“The longer it stays there, the longer it impedes light from reaching the vegetation,” said Francesco Cinelli, an ecology professor at the University of Pisa in Tuscany.