Mammoth cruise ships can be difficult to get around, even in the best of circumstances. In the worst - which is how one might describe the situation aboard the listing Costa Concordia - they are near impossible.

Yet even before the sun rose Monday morning, about 120 rescue personnel were out in or around the liner that hit rocks near Tuscany and rolled spectacularly on its side.

They were in a race against time, and in a battle with numerous challenges, to try to save or at least recover the bodies of the passengers and crew members who are still missing.

"(They are) working in very, very bad conditions," said Luciano Roncalli of Italy's national fire service. "It's cold, of course. It's dark during the day and the night... It's really, really dangerous."

Authorities have said that at least six people died after the Concordia hit rocks Friday night off the tiny island of Giglio, where nighttime temperatures have recently dipped below freezing.

Authorities are reviewing passenger lists to confirm the exact number of missing people, but about 16 are believed still unaccounted for.

Among them are two of the 120 Americans who were aboard the ship, the U.S. Embassy in Italy said.

Water, which makes many cruises serene and unique, has become rescuers' biggest obstacle.

Now turned on its side, the ship is roughly half submerged.

The hope is that a survivor has found refuge in a part that is not underwater, or perhaps in an air pocket, and can be brought out alive

In its current state, the Costa Concordia resembles a dark, convoluted cave - with its countless nooks and crannies and few ways to easily escape for air.

Rescue personnel working off the coast of Tuscany include about six underwater cave rescue divers.

They are likely equipped with twice as much oxygen as regular scuba divers, have a guideline nearby in case they need help finding a way back to safety, and have knives and whatever lights they can carry or wear, said Robert Laird, a co-founder of the International Underwater Cave Rescue and Recovery group.

Still, whatever equipment and precautions they take, "what they are doing is extremely difficult," he said.