After burning for a week on open water, an Iranian tanker that was carrying 136,000 tons of oil finally sunk into a part of the Pacific Ocean caught in a territorial tug-of-war between China and Japan.

The resulting oil spill has nearly grown to the size of Paris. The world, however, may never know its full impact, according to experts.

The Iranian tanker, the Sanchi, collided with a Chinese grain-carrying freighter on Jan. 6 in the contested East China Sea and caused an explosion that killed all 32 crew members. Some of the fuel burned off during the week the tanker stood flaming on the water’s surface. Just how much made it into the ocean, though, is anybody’s guess. So far, neither Japan nor China has taken charge of the clean-up or assessing the damage. To make matters worse, the Sanchi sunk on Jan. 14, burying much of the evidence deep underwater.

“I've not heard anyone — not the government of China or the government of Japan — have told me that they're conducting a major environmental evaluation,” said Richard Steiner, an oil spill and environmental specialist who’s advised countries like the U.S., Lebanon, New Zealand, and Japan as well as the United Nations. “As the regional governments never conducted a comprehensive monitoring program, we will never really know the impacts of this disaster.”

Neither the Chinese nor Japanese governments responded to a request for comment.

The burning Iranian oil tanker Sanchi is seen partially sunk in the East China Sea off the eastern coast of China on Sunday, Jan. 14, 2018. (Ministry of Transport via AP)

When the Sanchi crashed, the ship started leaking a type of oil called natural gas condensate, although we still don’t know exactly what type. Regardless, a condensate spill of this magnitude has never happened before. During the week the ship spent on fire, the spill nearly doubled in size.

When the Exxon Valdez sank in 1989, for example, the tanker spilled 10.8 million gallons of crude oil into the Prince William Sound off the coast of Alaska. While the spill caused huge amounts of damage, crude oil isn’t soluble in seawater and environmental clean-up efforts skimmed at least some off the surface.

Condensate, on the other hand, is mostly soluble in water. It won’t form a slick on the surface of the water the way crude oil does, and once it dissolves into the ocean, there’s essentially no recovering it, according to Steiner. The three slicks visible on the surface now are likely leftovers of the non-soluble heavy fuel used to power the ship that didn’t burn off.

“It’s typical for us to attend approximately 20 shipping incidents a year, and we’ve been doing this for 50 years,” Alex Hunt, a technical manager at the International Tanker Owners Pollution Federation, an organization that represents the oil shipping industry, told the Atlantic. “There have been other condensate spills, but this is the first condensate spill of this magnitude that we’ve dealt with, which gives you an impression of how rare cases like this are.”

Spilling condensate oil, however, could have been a blessing in disguise, according to Edward Overton, a professor of environmental science at Louisiana State University. While condensate is water-soluble, just how much dissolves depends on a number of factors. The rougher the seas, the more the condensate will dissolve. The oil could also simply rise to the surface and evaporate.

“The least impactful substance to spill is a condensate like this,” Overton said. “Since there’s not a high probability of a coastal impact, they’re probably going to let Mother Nature handle it.”

Regional governments, in fact, haven’t done a full environmental assessment of the spill yet. China has been conducting aerial surveys and taking water samples at the wreck site, according to the country’s daily reports. But Japan doesn’t appear to have mobilized a response at all. By now though, most of the oil would have drifted away from there with ocean currents, according to Steiner. A portion of the oil also burned off during the course of the week, further complicating efforts to determine how much spilled into the ocean. We just may never know.

Still, a few things could be done. If any oil tanks were aboard the Sanchi, they’re now at the bottom of the ocean and could be recovered. In 2004, two years after the oil tanker Prestige sank off the coast of Spain, for example, remotely operated underwater vehicles managed to recover 3.5 million gallons of oil trapped in the ships hull. The tanks, however, could also be breached and leaking.

China’s State Oceanic Administration, the agency responsible for coastal environmental protection, said officials would send robot-submarines down to inspect the ship on Thursday, according to emails obtained by VICE News. But it’s unclear whether the robot-boat has made it to the seabed, and, if it did, whether it found any unbreached cargo holds.