Much of the water 12 miles miles off the Gulf Coast is shallow, in some cases only five feet deep. That and the fact that this region experiences very passive tides, means that any oil gets beyond the protective boom barrier may be here to stay.
This poses a deadly threat to birds: Brown Pelicans, Laughing Gulls and the Least Tern. They once numbered 12,000 on the Mississippi Coast. There are now only about 2,000.
"It has the great potential of wiping out the entire population of Least Terns along the coast area," the director of wildlife care and rescue center director Alison Sharpe told CBS News special correspondent Jeff Corwin.
It only takes about 20 days for the eggs of the Least Tern to hatch and another 20 days for the chicks to leave the nest. This is the most critical period in the life cycle of these birds, when they're most vulnerable.
Here's what oil does to a bird: It causes feathers to mat and separate, making the birds more vulnerable to hypothermia and less waterproof and buoyant. When they preen their feathers, they swallow the oil, causing internal organ damage and eventually, death.
"The quickness with which we get to them and de-oil them and treat them will depend on whether they recover or not," says biologist Dr. Bill Walker.
Rescue teams on Sunday recovered more than 20 dead sea turtles. But tests on five of them so far show no evidence the oil killed them.
But for today, in these marshes, there was no trace of the poisonous slick that may be headed their way.