Part of an ongoing series on the environmental impacts of the Gulf oil spill.

If efforts fail to cap the leaking Deepwater Horizon wellhead in the Gulf of Mexico (map), oil could gush for years—poisoning coastal habitats for decades, experts say.

Last week the joint federal-industry task force charged with managing the spill tried unsuccessfully to lower a 93-ton containment dome (pictures) over one of three ruptures in the rig's downed pipe.

Crystals of methane hydrates in the freezing depths clogged an opening on the box, preventing it from funneling the spouting oil up to a waiting ship.

Watch video of the failed attempt to cap the leaking pipe.

Yesterday a smaller dome was laid on the seafloor near the faulty well, and officials will attempt to install the structure later this week.

But such recovery operations have never been done before in the extreme deep-sea environment around the wellhead, noted Matthew Simmons, retired chair of the energy-industry investment banking firm Simmons & Company International.

For instance, at the depth of the gushing wellhead—5,000 feet (about 1,500 meters)—containment technologies have to withstand extremely high pressures.

Also, slant drilling—a technique used to relieve pressure near the leak—is difficult at these depths, because the relief well has to tap into the original pipe, a tiny target at about 7 inches (18 centimeters) wide, Simmons noted.

"We don't have any idea how to stop this," Simmons said of the Gulf leak. Some of the proposed strategies—such as temporarily plugging the leaking pipe with a jet of golf balls and other material—are a "joke," he added.

"We really are in unprecedented waters."

Gulf Oil Reservoir Bleeding Dry

If the oil can't be stopped, the underground reservoir may continue bleeding until it's dry, Simmons suggested.

The most recent estimates are that the leaking wellhead has been spewing 5,000 barrels (210,000 gallons, or 795,000 liters) of oil a day.

And the oil is still flowing robustly, which suggests that the reserve "would take years to deplete," said David Rensink, incoming president of the American Association of Petroleum Geologists.

"You're talking about a reservoir that could have tens of millions of barrels in it."

At that rate, it's possible the Gulf oil spill's damage to the environment will have lingering effects akin to those of the largest oil spill in history, which happened in Saudi Arabia in 1991, said Miles Hayes, co-founder of the science-and-technology consulting firm Research Planning, Inc., based in South Carolina.

During the Gulf War, the Iraqi military intentionally spilled up to 336 million gallons (about 1.3 billion liters) of oil into the Persian Gulf (map) to slow U.S. troop advances, according to the U.S. National Oceanic and Atmospheric Administration.

Hayes was part of a team that later studied the environmental impacts of the spill, which impacted about 500 miles (800 kilometers) of Saudi Arabian coastline.

The scientists discovered a "tremendous" amount of oiled sediment remained on the Saudi coast 12 years after the spill—about 3 million cubic feet (856,000 cubic meters). (See "Exxon Valdez Anniversary: 20 Years Later, Oil Remains.")

Oil Spills Create Toxic Marshes

Perhaps most sobering for the marsh-covered U.S. Gulf Coast, the 2003 report found that the Saudi oil spill was most toxic to the region's marshes and mud flats.

Up to 89 percent of the Saudi marshes and 71 percent of the mud flats had not bounced back after 12 years, the team discovered. (See pictures of freshwater plants and animals.)

"It was amazing to stand there and look across what used to be a salt marsh and it was all dead—not even a live crab," Hayes said.

Saudi and U.S. Gulf Coast marshes aren't exactly the same—Saudi marshes sit in saltier waters, and the Middle Eastern climate is more arid, for example. "But to some extent they serve the same ecological function, which is extremely important," he said.

As the nurseries for much of the sea life in the Gulf of Mexico, coastal marshes are vital to the ecosystem and the U.S. seafood industry.

It's also much harder to remove oil from coastal marshes, since some management techniques—such as controlled burns—are more challenging in those environments, said Texas Tech University ecotoxicologist Ron Kendall.

"Once it gets in there, we're not getting it out," he said. (See pictures of ten animals threatened by the Gulf oil spill.)

Gulf Coast Should "Plan for the Worst"

Depth isn't the only factor that can stymie attempts to plug an oil leak.

The 1979 Ixtoc oil spill, also in the Gulf of Mexico, took nine months to cap. During that time the well spewed 140 million gallons (530 million liters) of oil—and the Ixtoc well was only about 160 feet (49 meters) deep, noted retired energy investment banker Simmons.

Efforts to contain the Ixtoc leak were complicated by poor visibility in the water and debris from the wrecked rig on the seafloor.

Also, the high pressure of oil in the well ruptured valves in the blowout preventer, a device designed to automatically cap an out-of-control-well. Recovery workers had to drill relief wells nearby before divers could cap the leak.

In general, Simmons added, officials scrambling to cap the Deepwater Horizon well should be working just as hard to protect the shorelines in what could become a protracted event.