Exclusive: Submarine Dive Finds Oil, Dead Sea Life at Bottom of Gulf of Mexico In exclusive trip aboard research sub, scientist shows how oil has sunk.

Dec. 3, 2010 -- A mile below the surface in the Gulf of Mexico, there is little sign of life.

"It looks like everything's dead," University of Georgia professor Samantha Joye said.

In an exclusive trip aboard the U.S. Navy's deep-ocean research submersible Alvin, ABC News was given the chance to observe the impact of this summer's massive oil spill that most will never see.

The ocean floor appears to be littered with twigs, but Joye points out that they are actually dead worms and that Alvin is sitting on top of what is considered an 80-square mile kill zone.

Having taken nearly two dozen dives in the Gulf inside the tiny sub that helped discover the Titanic, Joye is leading a team of scientists who are investigating how much oily material is left on the sea floor.

Aboard the Alvin Thursday, Joye said she saw "about three to four inches of material."

The devastation, she said, could last "years or decades."

"It's still there and it's going to degrade very slowly," she said.

That as BP, the company responsible for the spill, is challenging government estimates that 200 million gallons of crude spill from its runaway well. London-based BP now insists it's half that.

But 5,000 feet down, the oil appears to be everywhere. The government estimates that less than 25 percent of the oil remains, but these scientists say it's not gone, just settled at the bottom of the ocean.

And the work they are doing can be dangerous.

"You've got two boats out in the middle of the ocean meeting and then you're transferring people and gear, which is kind of sketchy to being with when it's calm like this," said the captain, Scott Sullivan, "but when you get offshore and it's a little rough, everything tends to be a lot more dangerous and on the water it happens a lot faster. So we're going to think safety first and teamwork second."

The boat that launches the Alvin -- the Atlantis -- is stationed some 50 miles offshore.

Alvin carries weights to drop to the sea floor, turning slowly as it descends. As the submarine drops, the pressure outside increases. And it is cold – the temperature of the near-freezing water. Everyone on board is bundled in layers of sweatshirts and blankets. The habitable part of the sphere is roughly the size of an car's interior.

But their work, they say, needs to be seen.

Gulf of Mexico Starting to Heal, but Effects of Oil Linger

The water in most places appears to be clear, a stark contrast to the oil that covered miles of ocean after the April 20 Deepwater Horizon rig explosion, which killed 11 workers.

But Joye said what was once on the surface has now sunk.

She told ABC News in September that samples taken from the ocean floor consistently show oil contamination.

"We're finding it everywhere that we've looked. The oil is not gone," Joye in September. "It's in places where nobody has looked for it."

But there are signs that the Gulf of Mexico is healing. Marinas deserted over the summer have become busy once again. With most of the area re-opened to fishing boats, captains and fisherman are hard at work.

Crews have also found healthier animals, especially the among the bird population around Grand Terre Island, La.