Air bubbles released from a pipe on the seabed will be used to form a buffer curtain to protect the marine park from noise pollution during the salvage.

A giant yellow drill shatters into the rocky seabed next to the rusting Costa Concordia wreck as workers battle to pull off the biggest salvage operation of its kind in history.

Cranes tower over the luxury liner, which lies covered in seaweed where it capsized on Giglio island in January. A gaping hole where the swimming pool used to be reveals the ghostly depths of the ship's nine-story central atrium.

The disaster, which killed 32 people, left salvage teams facing the unprecedented challenge of removing a ship with a gross tonnage of 114,500 GT without spilling its rotting contents into the sea.

"It's the biggest ship recovery ever by quite some way," said Nick Sloane, salvage master for US company Titan, which won a bid for the project jointly with Italian offshore rig company Micoperi to right and float the Concordia. "The plan is based on a lot of assumptions made by our engineering teams. It's a thumbsuck, but an informed thumbsuck," the South African said with a grin, adding that he has a cigar ready to celebrate the day the ship floats.

One of the biggest risks is that the ship, which is grounded on two large outcrops close to the shore, will slip when righted and plunge into the depths. The plan is for 26 pillars to be driven into the seabed to support a series of underwater platforms as big as football fields for the ship to sit on.

Large metal tanks that can be filled with water will then be welded onto the sides of the ship to balance the giant wreckage while it is dragged into an upright position using two cranes as well as cables attached to the platforms. The largest of the tanks are as high as an 11-storey building and weigh 500-plus tons, and getting them lined up precisely on the frame is far from easy.

"There's never been anything like this. It's part salvage, part offshore operation," said Franco Porcellacchia from the ship's parent company Carnival.

Teams working late into the night this week at the operation's nerve center in a hotel on Giglio coordinated the arrival of 66 divers tasked with putting 17,500 tons of cement bags in a 50-metre gap between the ship and the seabed.

The capsized Costa Concordia cruise ship lies off the port of Giglio Island, Italy.Corbis

The fear is that the midship section may give way, breaking the ship in half -- a risk greatly increased should there be bad weather over the coming winter.

"She's spanned between the reefs so the more support you give her the better she'll survive the winter. If we have a mild winter that will be great, but it's unlikely, and bad weather will mean delays for sure," Sloane said.

The project, which Carnival says will cost at least 400 million euros ($525 million), is already running several months late due to technical issues.

"The seabed is granite rock, not limestone or sandstone," Sloane said. "Granite rock is the worst kind to be drilling in, especially at the 35 to 40 degree angles that we're drilling. The drill head also slips on the rock." The team finally managed to get the first hole drilled this week.

Local builder Luca said he didn't mind how long it takes: "I'd rather they take their time over it than rush it, break the ship and pollute the shoreline" where even in mid-October locals and tourists swim in the crystal-clear waters.

Life in the marine park is being monitored by Giandomenico Ardizzone, professor in environmental biology at Rome's Sapienza University, whose 15-man team have been painstakingly saving rare giant mussels from under the wreck.

He also has watchers go out in a boat twice a day to see whether there are any whales or dolphins near the Concordia: "If there were, the drilling would stop because it could damage their hearing even over a long distance."

And to limit the intense vibrations and din when the wreck is righted, the professor is drawing up plans for a bubble wall created by air bubbles released from a pipe on the seabed to form a buffer curtain.

"It's the first time a system this complicated will have been used for salvage," he said. "The plan is not only to reduce noise but also create a barrier to trap pollution if the stagnant water inside the ship should spill."