Oil from a wrecked tanker is creating a disaster in the waters of Bangladesh's Sundarbans, the largest contiguous tidal mangrove forest in the world and a haven for a spectacular array of species, including the rare Irrawaddy and Gangetic dolphins and the highly endangered Bengal tiger.

"This catastrophe is unprecedented in the Sundarbans, and we don't know how to tackle this," Amir Hossain, chief forest official of the Sundarbans in Bangladesh, told reporters.

Named for the native sundari tree, the Sundarbans is a vast delta of densely forested, mangrove-fringed islands threaded by an intricate network of creeks and channels, or canals. The delta is a UNESCO World Heritage site that encompasses some 3,850 square miles (1,000 square kilometers), with roughly one-third lying in India and two-thirds in Bangladesh.

On both sides of the border, densely populated villages abut protected areas set aside for the region's extraordinary biodiversity.

View Images NG Staff. Sources: UNEP-WCMC; ProtectedPlanet.net

In the early hours of December 9, in dense fog, the tanker Southern Star 7, carrying some 92,000 gallons of bunker oil, was rammed by a cargo vessel in the Sela River, at the entrance to the Bangladesh Sundarbans, southeast of the river port of Mongla. The collision occurred inside the Chandpai dolphin sanctuary.

Seven crew members survived by jumping ship and swimming to shore. The body of Captain Mokhlesur Rahman was retrieved five days later.

Reportedly, some 52,000 gallons of fuel has already leaked into the brackish tidal water. According to Hossain, the spill has spread over a 40-mile-long (64 kilometers) area along the Sela and Pusur Rivers.

Official announcements have not yet clarified the extent of the damage. News footage shown on local television has revealed lines of oil-blackened shoreline and mangrove trees bearing a high-tide mark of black oil.

In television interviews, villagers and fishers have complained of the smell of oil. Sightings of dead fish and crabs have been reported in the Chandpai region's channels.

View Images This aerial photograph of the Sundarbans, made in 2010, shows a densely forested landscape interlaced with creeks and channels. Photograph by Jonas Bendiksen, National Geographic

A Place of Wild Beauty

Remote and hard to get to except by water, the Sundarbans is a place of wild, menacing beauty. Here, jewel-like kingfishers may perch above a sleeping estuarine crocodile or fly over the fin of a passing shark.

At low tide, otters, monkeys, wild boars, and spotted deer emerge from the forest, and the mud banks regularly bear the deep pugmarks of a striding tiger.

Bird life is famously prolific, with more than 300 different species known to live in the area.

In 2011, the New York-based Wildlife Conservation Society discovered a remarkable population of 6,000 Irrawaddy dolphins in the Bangladesh Sundarbans.

The find led to the creation of three sanctuaries for Irrawaddy and long-nosed Gangetic dolphins. The fact that the oil tanker was wrecked in one of the sanctuaries has heightened the grave concerns about the environmental impact.

View Images In the Sundarbans, endangered tigers are at home in the mangrove forests. Tigers are good swimmers, which helps them get around in this watery region. Photograph by Steve Winter, National Geographic Creative

Life-Sustaining Mangroves Threatened

Fringing the land with long, twisted roots, mangrove trees have evolved to withstand the relentless tugging of a powerful tide that alternately swamps the low-lying forest and recedes to leave glimmering sand.

The tide constantly reshapes channels and shifts sand, creating new banks for the many creeks and rivers that the mangrove is adept at colonizing. At the edge of the water, the mangrove's sprawling root system is the land's front-line defense against erosion.

The many species of mangrove that make up the Sundarbans' forests are central to the region's intricate chain of life.

For humans, the mangroves provide wood and tannin, which is used in medicine. Birds shelter in their foliage, while their fallen leaves feed shrimp and other marine life attached to the mangroves' scaffold-like roots. On land, insects burrow and feed in the water-softened wood.

Famously, the mangrove forest holds one of the last major tiger populations on Earth. More than a hundred tigers live in the Indian Sundarbans, and an unknown number in the less-studied, larger Sundarbans of Bangladesh.

With only an estimated 3,000 wild tigers left throughout Asia, a population of even a hundred is hugely significant.

The Sundarbans tigers are powerful swimmers and supplement their diet of land prey with marine life, including crabs.

According to Alan Rabinowitz, CEO of Panthera, an organization dedicated to saving big cats, the tiger's adaptation to the difficult Sundarbans terrain is a testament to the remarkable resilience of the species. "To be surviving on brackish water, to navigate these mangrove swamps and mud—these are not easy adaptations for any mammal," he said.

View Images On December 12, a fisher does what he can to scoop up oil fouling the water not far from where a cargo vessel struck the tanker Southern Star 7. Photograph by Reza Shahriar Rahman, Counter Foto

Disaster Response Criticized

Criticism of the Bangladesh government's slow response to the unfolding disaster has been fierce.

Two days passed before the leaking tanker was towed to land, and the spill is reported to have already drifted into numerous channels leading out of the Sela River.

Little confidence has been expressed in the cleanup efforts now under way, which appear to be relying on local people.

The company that owns the tanker is using buoys to bar the oil slick from spreading and has initiated a "buy back" program, paying villagers 30 taka (less than 40 cents) for every liter of sludge retrieved. Local fishers have been asked to scoop up the oil with their nets.

As one environmental scientist said, "Possibly, there is no expertise to handle an oil spill in an estuary in this part of the world."

How a shipping route of any kind was allowed to pass through all three of the dolphin sanctuaries is one of the many uncomfortable questions officials will likely be asked in the days ahead.

Across the border in India, officials are closely watching the ongoing disaster.

According to Pradeep Vyas, director of the Sundarbans Biosphere, a protected area within India's Sundarbans National Park, "wildlife officials have been deployed along the Sundarbans area bordering Bangladesh to check if the spill is spreading."

The fact that the accident happened in the low-water season, rather than the rainy season, will limit the oil's spread inland.

"There will be local disruptions affecting crustacean population[s] more than any other group in the short term," according to Anurag Danda, head of the Sundarbans Program Office for WWF India.

"In the short to medium term, plants with pneumatophores will struggle too," he said, referring to the spiky respiratory roots of the mangrove tree that rise, snorkel-like, through the mud.