What could be the largest oil spill since 1989's Exxon Valdez is unfolding in the East China Sea after a deadly and fiery collision between two vessels caused a tanker to sink. All 32 crew members are thought to have died aboard the Iranian vessel "Sanchi," which was carrying about 1 million barrels of condensate.

According to Bloomberg News, the ship was transporting hydrocarbon liquid that's a key ingredient for making petrochemicals, including jet fuel. It was headed to the port of Daesan, South Korea when it struck the transport ship "CF Crystal" off China's eastern coast.

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The tanker and its associated oil slick had been on fire for days after the collision. While the fire likely killed all aboard the ship, it was seen by environmental experts as a way to minimize the broader impacts of the spill, since the flames burned off the lightweight condensate on the ocean surface.

However, the fire is now out, and the ship has sunk, raising the possibility that the harmful cargo is going directly into the sea.

The cargo is different than the crude oil spilled by the Exxon Valdez in 1989, but if all the condensate were to leak into the ocean, it would rank as the biggest spill in decades.

Much remains unknown about the fate of the cargo, and therefore similar can be said about what the environmental impacts will be. Reports in recent days are not encouraging, since there is word of a rapidly spreading oil slick on the surface of the ocean. Citing Chinese authorities, Bloomberg reported that the spill expanded from 3.9 square miles to 52 square miles between Sunday and Monday local time.

An oil spill in the heavily trafficked East China Sea could have significant environmental repercussions. Humpback whales travel through that area, and heavily fished species such as mackerel and bluefin also spend time in that area.

“It is virtually certain that much of the condensate went into the sea in solution, and that toxic underwater hydrocarbon plume will injure marine life exposed to it,” Richard Steiner, an oil spill specialist based in Alaska, told Bloomberg. “Even the burned fraction will leave a toxic residue on the water.”

Ma Jun, a Chinese environmentalist, was quoted by CNN as saying the spill took place in one of the most productive fishing areas in the country, known as the Zhoushan fishing ground.

A handout photo made available by the Transport Ministry of China shows smoke rising from the fire on the Panama-registered tanker 'Sanchi' on Jan. 14, 2018. Image: TRANSPORT MINISTRY OF CHINA HANDOUT/EPA-EFE/REX/Shutterstock

"We still need to keep an eye on how these contaminants might be carried by the ocean flow to have the impact on the fishing ground," Jun told CNN.

According to Greenpeace International, it's not clear how large this environmental disaster will be, since the amount of condensate that leaked into the water is unknown.

"A major concern is that, now that the tanker has sunk, any condensate which did not yet burn off could continue to leak underwater, disperse and break down quite quickly, significantly complicating clean up operations," the environmental advocacy organization stated in a Jan. 15 fact sheet.