DULAC, La. – Claude Luke throttles down his 21-foot aluminum work boat. Off to the left, the snout of an alligator disappears near the mouth of a watery gash in the Louisiana marshland.

The 51-year-old Cajun crab fishermen is touring the epicenter of an unfolding environmental disaster that dwarfs the BP spill and predates it by decades, according to state scientists and environmentalists.

If unchecked, the destruction threatens to undermine the world’s seventh largest estuary and one of the nation’s most important energy corridors.

His boat idles near a canal dredged more than two decades ago for a petroleum pipeline. Back then it was about 15 feet wide. Now it sprawls 100 feet wide, opening this once-protected upland marsh to toxic salt water.

Not far away, Luke nods toward a water tower visible across about two miles of almost open water.

“You used to be able to walk there from here,” says Luke, who moonlights as a warden on the private Harry Bourg Corp. preserve deep in Louisiana’s delta. “Before the oil companies came, this was good, solid marsh.”

More than half the 17,000 acres of marshland purchased about 80 years ago by Bourg, a barely literate muskrat trapper, have been lost to erosion and subsidence, according to engineering surveys.

The inheritance of Bourg’s descendants is vanishing under a profusion of these runaway canals. They were dug to lay pipelines or float in equipment for the drilling of 90 oil and gas wells that made Bourg one of the wealthiest men in South Louisiana before he died in 1963.

Long before BP’s blowout menaced the Gulf of Mexico, an oil industry-related coastal crisis of another kind began unfolding all over the Mississippi River coastal delta.

Between 1901, when drilling began in Louisiana, and the 1980s, the oil and gas industry laid tens of thousands of miles of pipelines and dredged 9,300 miles of canals in an industrial invasion of a wetland that once covered 3.2 million acres.

Since the 1930s, more than a third of it has vanished, an area the size of Delaware. Each year, 15,300 more acres disappear, according to Louisiana’s Comprehensive Master Plan for a Sustainable Coast.

No single culprit

Not all this can be blamed on oil and gas drilling; the industry rejects the notion that it is chiefly responsible. Whatever the case, the destruction of marshland reverberates far beyond Louisiana.

The state’s waters and wetlands underpin a commercial seafood industry that generates about $2.4 billion a year in wages and sales and provides almost a quarter of the catch in the contiguous U.S., according to the Louisiana Seafood Marketing Board.

They serve as wildlife breeding grounds, sheltering and feeding 5 million migratory birds a year, according to state data. The wetlands also absorb and filter out pollutants and help slow storm surges.

The latter is important because coastal Louisiana accounts for 27 percent of U.S. energy production. And an 83,000-mile infrastructure of pipelines and transfer stations transports 40 percent of its energy needs – counting petroleum from imports and offshore wells – according to data from the state’s Department of Natural Resources and the Louisiana Mid-Continent Oil Gas Association.

The collapse of Louisiana’s coastal marshes is “an international economic and ecological calamity unequaled in history,” jeopardizing more than “$100 billion in energy infrastructure,” said America’s Wetland Foundation, a Louisiana coastal preservation group partly underwritten by the oil industry, in a 2008 report.

Much of the pipeline network is buried beneath marshes. Erosion has already exposed high-pressure pipelines to storms and marine traffic, causing oil spills and accidents.

The state has floated an ambitious marsh and barrier-island rebuilding program that, since Katrina, it has tied to hurricane protection. The cost may come to $50 billion over time, according to the plan.

“The oil industry damage to the Louisiana coast via oil and gas exploration over the past 80 years dwarfs the BP spill and all other spills past, present and future,” says Oliver Houck, director of the environmental law program at Tulane University in New Orleans.