Large amounts of solidified, lava-like, rocks in layers as much as 3 feet (1 meter) thick were seen under Fukushima's Unit 3 reactor, according to Tokyo Electric Power Co. (TEPCO), which operates the plant.

"It's natural to think that melted material flowed out from the reactor pressure vessel," a company official told a news conference late Saturday.

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Three of the plant's four reactors were completely destroyed in 2011 by an earthquake-triggered tsunami that slammed into the waterfront power station. The fourth reactor was also badly damaged.

The 30-centimeter-long remote-controlled robot, which is equipped with two cameras made the discovery on Friday, during the second day of its three-day probe of Unit 3.

Extremely high radiation levels

Images from the robotic probe showed extensive damage to the unit caused by the core meltdown. Fuel debris was seen mixed together with broken reactor parts, suggesting that the decades-long decommissioning process that lies ahead will be difficult.

TEPCO spokesman Takahiro Kimoto said more time is needed to fully analyze the images to figure out how to remove the debris.

Almost 6.5 meters (21 feet) of water has accumulated at the bottom of the containment vessel, as the operator continues to inject water into the unit to cool the fuel.

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Locating and analyzing the fuel debris and damage in each of the plant's three destroyed reactors is critical for decommissioning the plant. So far the search for melted fuel in the other two reactors has been unsuccessful because of damage and extremely high radiation levels.

Earlier this year, the operator sent a robot into reactors 1 and 2, but it was not able to detect fuel debris at the units.

bik/rt (AP, dpa)