Bloomberg/Getty Images An employee walks past storage tanks for contaminated water at the Tokyo Electric Power Co.'s Fukushima Dai-ichi nuclear power plant in Okuma last February.

In a potentially major milestone in the cleanup effort of Japan’s Fukushima nuclear plant, an underwater robot has captured what’s believed to be the first images of melted nuclear fuel inside one of the plant’s three damaged reactors.

The remote-controlled robot, nicknamed “Little Sunfish,” found “large amounts of solidified lava-like rocks and lumps” at the bottom of Fukushima’s Unit 3 reactor over the course of a three-day investigation that ended Saturday, reported the Associated Press.

The debris still needs to be analyzed, but the plant’s operator, Tokyo Electric Power Company (TEPCO), seemed confident that the objects were fuel. A spokesperson said Friday there was a “high possibility that the solidified objects are mixtures of melted metal and fuel,” according to the BBC.

If true, the discovery could mark a watershed moment for the complex and costly cleanup of the Fukushima plant — an effort that could cost more than $70 billion and up to 40 years to complete, the Japan Times reported.

Underwater robot locates possible melted fuel inside #Fukushima reactor - a key step in the decommissioning process https://t.co/sPaRX8pItK pic.twitter.com/RgSJcpcHPG — AFP news agency (@AFP) July 23, 2017

Finding, analyzing and removing the melted fuel in Fukushima’s damaged reactors is one of the most critical steps in the cleanup and decommissioning of the plant, the AP noted. The melted fuel has rendered the reactors so radioactive that a human being could be killed within minutes of nearing one.

The search for the molten fuel had long been stymied, however, as extensive damage and staggering levels of radiation have proven insurmountable even for some of the robots invented to probe Fukushima ― some of which have “died” while attempting to approach the facility’s reactors.

TEPCO’s Naohiro Masuda, the head of the Fukushima decommissioning effort, said that radiation had destroyed the wiring on many robots, leaving them incapacitated.

“It is extremely difficult to access the inside of the nuclear plant. The biggest obstacle is the radiation,” Masuda told Reuters last year.

TORU YAMANAKA/Getty Images Little Sunfish, a submersible robot developed by Toshiba and a team of researchers, was "designed to tolerate radiation of up to 200 sieverts, a level that can kill humans instantly," reported the AP.

Little Sunfish, created by the Japanese company Toshiba and a team of nuclear researchers, was not just able to survive the reactor but emerged with the potentially critical images.

The robot’s expedition “might be evidence that the robots used by TEPCO can now deal with the higher radiation levels, at least for periods of time that allow them to search parts of the reactor that are more likely to contain fuel debris,” M.V. Ramana, a physicist and Fukushima expert, told Japan Times. “If some of these fragments can be brought out of the reactor and studied, it would allow nuclear engineers and scientists to better model what happened during the accident.”

The Fukushima disaster, triggered by a massive earthquake and tsunami in March of 2011, was one of the worst nuclear accidents in history.

Since the disaster, some sections of the facility were restored and thousands of employees now work on the site, conducting cleanup work, according to a Los Angeles Times report last year. In 2014, one of these employees was diagnosed with cancer ― a diagnosis that officials recognized as being linked to his work at Fukushima.

TEPCO said this week it is working on a plan to remove the melted fuel from the facility’s reactors. The company hopes to begin the removal process in 2021.