Thanks to an underwater robot nicknamed “Little Sunfish,” authorities in Japan may have reached a critical stage in the cleanup effort following 2011’s Fukushima disaster.

Late last week and this weekend the unmanned craft poked through the ruins of one of the doomed plant’s three reactors, finally sighting what might be melted fuel collected in clumps and icicle-like formations around the wreckage. If so, it would be a key moment in the decontamination.

“From the pictures taken today, it is obvious that some melted objects came out of the reactor,” Takahiro Kimoto, a spokesman with the Tokyo Electric Power Company, said Friday, according to the Japan Times. “This means something of high temperature melted some structural objects and came out. So it is natural to think that melted fuel rods are mixed with them.”

He said it was “highly likely” to be melted fuel.

“The pictures that we have gained will assist us in devising a plan for removing the melted fuel,” Kimoto said, according to Bloomberg. “Taking pictures of how debris scattered inside of the reactor was a big accomplishment.”

Triggered by a massive earthquake and tsunami that rocked the Japanese coastline March 11, 2011, the reactor disaster at Fukushima was the worst nuclear incident since the 1986 Chernobyl disaster. Three of the plant’s reactors plunged into 20 feet of water, releasing enough radioactive fallout to force 200,000 to evacuate their homes.

Six years later the area remains highly contaminated — so much so only specially designed unmanned robots can explore the region. But the lack of concrete information has been a serious handicap for the cleanup up effort: Corium, the lava-like refuse from a nuclear meltdown, must be located before decontamination begins.

So far, the search for melted fuel at Fukushima has not gone well. According to Reuters, previously unmanned crafts have not survived the intense radiation. And previous dives by “Little Starfish” on the two other reactors did not yield any visual confirmation.

But on Friday, the reactor’s owners announced that the probe’s footage showed evidence of what might be the melted fuel. Further analysis will be required to be certain.

With the location of the contaminants possibly identified, the work is just beginning. The Japan Times reports the utility hopes to start the removal process in 2021. Last year the Japanese government released an estimate saying it could cost $72 billion and 40 years to decommission the reactors.

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