The airboat signs on Highway 41 start just west of Miami, where the stucco sprawl of Little Havana gives way to South Florida's vast, mucky wilderness of saw grass, mangroves and cypress trees.

Beckoned by promises of swamps and alligators, you can pull over and pay $20 to glide for a while on an airboat, flat-bottomed and wide, propelled by a huge fan so noisy that guides hand out cotton for your ears. Wherever you stop, the view is essentially the same: marshland so flat and expansive you can almost tell the world is round, and sense its limits. You may see snapping turtles, egrets and herons that look almost prehistoric and, of course, alligators, lured into camera range by marshmallows, which the guides keep tossing until the film runs out.

The wind rustles the tall grasses in this landscape obstructed only by occasional tree islands, held sacred by the Miccosukee Indians, who still inhabit parts of the Everglades. The only other sounds are the soft splashes of fish jumping, the calls of wading birds and the beating of their wings, and the chirring of insects.

Take off your shoes and climb into the shallow muck for a moment -- some guides will stop to let you, well before the marshmallows come out -- and it feels oozy and primeval and mysterious between your toes.

This is about as deep as anyone goes into the Everglades, one of the world's most unusual ecosystems and the battlefield in one of history's costliest and most protracted environmental wars.

Everyone has heard of this place. But even those who live and work on its fringes tend to regard it from a distance, as if a region so forbidding, wrapped up in political struggles so old and so complicated, can only defy understanding.

The background is simple enough. For thousands of years, water from 730-square-mile Lake Okeechobee, just north of the Everglades, spilled south in lazy sheets toward the ocean. That constant, slow coursing created marshy rivers and saw-grass prairies that nurtured myriad species of birds, snakes and fish, plus deer, panthers, alligators, bears and manatees.

But the fate of this soggy habitat changed significantly in the boom years after World War II, when homeowners in new subdivisions demanded drinking water and flood protection. At the same time, Florida's growing sugar industry started taking over what had been the Northern Everglades (now formally designated the Everglades Agricultural Area) and craving water for farming. So engineers constructed a maze of canals to divert much of the lake overflow toward the cane fields and away from newly populated areas. During seasons of heavy rain, like the unusually wet summer that just passed, the canals also keep the sugar fields from flooding.

As a result of all this, the Everglades -- which comprise some two-thirds more land than the 1.5 million acres within Everglades National Park -- have been shrinking for decades. They are now about a fifth of their original size, and much of the shrinkage has occurred in the last 50 years.

A great deal of the water that still reaches the Everglades is polluted with phosphates from agricultural and household runoff, spawning cattails that choke the marshes and crowd out native plants and animals. The wading-bird population is only 10 percent of what it was in the late 1800's. Many species are disappearing, including the Florida panther, the wood stork and the Cape Sable sparrow.

Other than scientists and environmentalists, it is hard to tell who cares. The drawn-out, convoluted Everglades drama seems to have captured few imaginations in South Florida, whose state of mind is far more attuned to sun and surf than to buggy marsh. ''Most people think it is a yucky, mucky place,'' said Lyle Thomas, president of Loxahatchee Everglades Tours, which he started as a ''responsible'' alternative to the sometimes reckless airboat operators along Highway 41.

People like Mr. Thomas have their theories about why so many Floridians do not bother with the Everglades. The low, monotonous landscape lacks the majesty of the Grand Canyon or Yellowstone, they say, which helps explain why only 1.1 million people visited Everglades National Park last year, compared with seven million at Walt Disney World. Park attendance peaked in the 1970's, at 1.7 million; surveys show that visitors rarely come back. The Everglades are confusing, even frightening: many people associate them with the grisly ValuJet crash of 1996 or, rightly, with mosquitoes and poisonous snakes.

Ask Mr. Thomas to explain the Everglades, and he'll let you know that you are in for a long conversation. ''Do you want the facts that were, the facts that might be tomorrow, or the facts that are here today?'' he asked.

Some unchanging facts are these: The federal government sued Florida in 1988 to stop polluted water from harming the Everglades. Sugar companies intervened, and a settlement that included a cleanup plan was reached in 1994. That year the State Legislature passed the Everglades Forever Act, which established cleanup rules not just for federal land, but for the entire Everglades area.