DELTA, B.C. (CP) - A large portion of Greater Vancouver on Tuesday resembled Los Angeles on a bad day: poor visibility, heavy haze and noxious fumes.

Only the problem in Vancouver was smoke, not smog, from the forest and bog fire in the ecologically sensitive Burns Bog. The bog, important enough to have its own society watching out for it, is about 25 kilometres southwest of downtown Vancouver, in a municipality known as Delta and bordered by the Fraser River and the Pacific Ocean.

The Burns Bog Conservation Society says the bog covers more than 40 square kilometres and is the largest undeveloped urban land mass in North America.

It is believed to be the largest domed peat bog in North America and one of the largest in the world. It is also sometimes called the "lungs" of Greater Vancouver because the bog acts as a kind of biological filter, helping to clean the air over a wide area.

The society arranges tours through the trails and waterways of the area, which is also home to more than 200 species of migratory birds who use the bog for resting, staging, and breeding.

But now the trails are closed and the wildlife is threatened as aircraft and helicopters make endless passes over the burning area, dropping thousands upon thousands of gallons of water onto the area.

The blaze started Sunday afternoon and when it is completely out is anybody's guess since the sphagnum moss that makes up the bog can burn underground for weeks or months.

Delta fire chief Gordon Freeborn said the attack tactics had changed already, with helicopters and ground crews in use Tuesday with the large Mars water bomber grounded.

"When the fixed wing aerials are working (dumping retardant) we can't have ground crews working down there," Freeborn told reporters gathered at a command post on the edge of the vast bog.

"(Monday) was successful in cooling things down so we can get people in there today on the ground."

The area that has either burned or is still burning encompasses about 200 hectares, or two square kilometres.

While the bog is uninhabited, the areas to the north, south and east are home to hundreds of thousands of people in several municipalities.

In Delta, Burnaby, New Westminster, Coquitlam and several others, the smoke was so thick as to limit visibility to a few blocks.

Anyone who has stood next to a campfire and had the smoke blowing into your face, knows the acrid smell.

Freeborn said the tactic change was "to get the water right in to the ground, to get each individual working a small area with a hose line and shovel and pick."

The fire chief, who had about 80 personnel working from his fire department, the Forests Ministry and the Greater Vancouver Regional District, said he hoped the crews would have the fire in a "mopup position" in about two weeks "and feel comfortable it's not going to flare up again."

Seven helicopters, including a massive Sikorsky dropping about 9,000 litres of water with each pass, worked the fire.

"The advantage of helicopters is they are more conducive to working with the ground crews. They can drop spot dumps and we don't have to pull crews out to work with them."

Last year, the federal, provincial and local governments came to an agreement that allowed a large piece of the bog to be protected as an Ecological Conservancy Area.

The bog has caught fire before, most recently in 1996 when it took two days to extinguish a blaze that also destroyed almost two square kilometres of the wetland.

Eliza Olson, president of the Burns Bog Conservation Society, said the blaze began in a corner of the bog where crews had begun lowering the water table to dry the area out.

She said a farmer was threatening to sue the city if his farmland became flooded.

Some facts on the Burns Bog, which has been hit by a wildfire just outside of Vancouver:

Size: At 40 square kilometres, the bog is 10 times larger than Vancouver's famed Stanley Park and is located on the south arm of the Fraser River.

Definition: Bogs are peat-accumulating wetlands that support plants that are able to thrive in waterlogged, acid and nutrient-poor conditions. Over thousands of years, sphagnum mosses, leaves and roots accumulate faster than they can be decomposed, forming peat.

Key Habitat: Twenty-four species of mammals including beaver, Columbian black-tailed deer, and the endangered southern red-backed vole live in the bog.

Rare Species: There are a variety of rare insect species in the bog, including two sub-arctic species of the Blue Darner dragonfly. Also living in the bog is the rare Pacific water shrew and about 200 species of birds, including the great blue heron, bald eagles and the threatened Greater Sandhill crane.

Plant Life: The bog contains stunted lodgepole pine, bracken fern, Labrador tea, bog laurel, skunk cabbage and sphagnum moss.

Lost Wetland: About 40 per cent of the original bog area has been lost to development and more than half the remaining bog has been disturbed by human activities.

National Wetlands: Only 17 per cent of Canada's original wetlands remain.