Now the clean-up begins as the burning Iranian tanker sank in the East China Sea.
A 46-square-mile oil slick consisted of heavy fuel used to power the vessel.
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The Sanchi oil tanker sank Sunday and officials said all its crew members died in the incident.
The ship was carrying 136,000 metric tons of ultra-light crude oil from Iran which generates a toxic underwater slick that would be invisible from the surface.
Between the fuel and the ultra-light oil, it could cause devastating damage to marine life.
The Sanchi and a cargo ship collided 160 miles off Shanghai on January 6, with the tanker then drifting south-east toward Japan.
It caught fire after the collision and burned for more than a week before sinking off China’s east coast.
Iranian officials said all 32 crew members – 30 Iranians and two Bangladeshis – on the tanker were killed.
On Monday, China Central Television said a search and rescue operation had been cancelled and a clean-up operation had begun after a fire on the surface was extinguished.
They said two ships were spraying the water with chemical agents designed to dissolve the oil.
The BBC’s China correspondent Robin Brant said the oil slick more than doubled in size since Sunday.
The big concern now is for the environmental impact, he said. There could also be a very tall plume of condensate, this ultra-refined form of oil, underneath the surface.
Condensate, which creates products such as jet fuel, is very different from the black crude that is often seen in oil spills.
It is toxic, low in density and considerably more explosive than regular crude.
Some 13 vessels and an Iranian commando unit took part in the salvage operation, amid bad weather.
On Saturday, salvage workers boarded the vessel and found the bodies of two crew members in a lifeboat.
Only one other body had been found during the week of salvage operations.
The rescue workers also retrieved the ship’s black box but had to leave quickly because of the toxic smoke and high temperatures.
The cause of the collision is still not known.