BP Oil Spill: Containment Cap to Be Tested, Environmental Devastation Spreads

July 13, 2010— -- As BP begins to test the new cap that the company hopes will finally contain its gushing leak in the Gulf of Mexico, crude oil continued to roll into sensitive coastal areas today, an assault on the environment that will last long after the leak is finally under control.

New images taken beneath the surface of the Gulf show plumes of oil, choking the sun from penetrating the surface and starving plants of a crucial element for life. Above the water's surface, experts say that sludge will continue to move onto shorelines for months and maybe even longer.

Watch 'World News' for more of Diane Sawyer's coverage of the oil spill.

"The concern is that the oil will go and enter the sediment," said Moby Salangi, a scientist researching the effects of the spill in the marshes of Mississippi.

The Gulf mud could act like a vault, locking the oil in and making it nearly impossible to clean up. The muddy soil is crucial for life that forms the base of the food chain, as it's the environment where plankton and fish larvae are incubated.

In Alaska, oil can still be found in the soil along the coastlines where the Exxon Valdez spill hit over 20 years ago.

Scientists say the magnitude of this spill, up to 20 times the size of the Exxon Valdez disaster, is so great that they can't yet measure the impact on Gulf animals like sea turtles, fish, shrimp and dolphins. Already, the toll is tremendous.

"As of just a few days ago 643 sea turtles have been found in the area of the spill. 464 of which were found dead or died in rehabilitation," said Charlie Henry, an environmental scientist for NOAA. "Sixty-two Dolphins have been found in the area of the spill, 60 of which have been found dead or died in rehabilitation, and there's been one sperm whale found beached in this response."

"This is an experiment actually happening," said Salangi. "We have never seen anything like this, and again, we don't understand how this will all recover."