This post has been updated.

Four days on, rescue crews are still struggling to reach the Sanchi, an oil tanker on fire in the East China Sea.

The tanker had collided with a South Korean freight boat on Saturday. Since then, an international effort has been launched to reach the Sanchi and put out the fire.

Already, the rescue crews have battled beating rains, toxic gas and 10-foot waves to get near the vessel. On Wednesday, an explosion on the tanker forced a retreat. According to the Chinese Transport Ministry, the blast occurred while rescue crews doused the ship with foam to try to put out the flames. It's unclear what kind of damage the explosion caused.

Thirty Iranians and two Bangladeshi nationals were aboard the tanker when the collision occurred. One body has been recovered but not publicly identified. Iranian officials say they are still hopeful that the crew is camped out, alive, somewhere safe — like the engine room. But experts are skeptical.

Since the crash, the Sanchi has been billowing thick plumes of black smoke. Unless the fire can be brought under control, officials worry that an explosion might sink the ship, releasing its 1 million barrels of oil (which amounts to 42 million U.S. gallons) into the water.

The resulting spill would be about three times as big as the Exxon Valdez spill of 1989, one of the worst environmental disasters in history. The oil spilled would be double what the Prestige tanker released when it sank off the coast of Spain in 2002. That accident damaged beaches in France, Spain and Portugal, and led to the closure of one of Spain's richest fishing areas.

(Some of the worst spills in history have been even bigger. When the Deepwater Horizon drilling rig exploded off the southern coast of the United States in 2010, it spilled about 210 million gallons of oil into the Gulf of Mexico. In 1979, the Atlantic Empress and the Aegean Captain collided, resulting in a 90-million-gallon oil spill. A 1991 explosion aboard the tanker ABT Summer off the coast of Angola spilled about 80 million gallons.)


A Chinese firefighting vessel sprays water on the burning oil tanker Sanchi at sea off the coast of eastern China. (AFP via China's Transport Ministry)

The Sanchi was transporting oil from Iran to South Korea on Saturday when it ran into the CF Crystal, a Hong Kong-registered ship carrying grain from the United States. The crash occurred about 160 miles off the coast of Shanghai and near the mouth of the Yangtze River. The cause remains unknown.

Experts are especially worried because the ship is carrying condensate, an ultralight version of crude oil. Condensate is highly toxic and even more combustible than regular crude oil. It also is nearly colorless and odorless, which makes it difficult to detect.

“This stuff actually kills the microbes that break the oil down,” Simon Boxall of the National Oceanography Center at the University of Southampton told the BBC. “If she sinks with a lot of cargo intact, then you have a time bomb on the sea bed which will slowly release the condensate.”


Smoke is seen from Panama-registered Sanchi tanker. (Reuters via China's Transport Ministry)

An oil leak into the East China Sea also could have a serious effect on the waterfront's wildlife. The East China Sea, where mackerel and croaker abound, is China's largest fishing ground.

If the ship does not sink, the environmental impact would be much more limited. The Chinese government said it has not found evidence of a “large-scale” oil leak. Much of the oil — between 40 percent and 70 percent — would probably evaporate in hours. Of course, if those toxic fumes drift toward towns and cities, they could aggravate existing health conditions or lead to coughing and asthma.

“The poisonous gas . . . is very harmful to rescue workers in the field,” according to a statement from the Shanghai Maritime Bureau.

The accident occurred in an area known as the “new Bermuda triangle” because it is so dangerous. At least 33 ships were lost in the area in 2016, according to Die Welt.