The operator of the damaged Fukushima Daiichi nuclear power plant released video footage on Monday of what is likely to be melted fuel debris.
Tokyo Electric Power Company, or TEPCO, found the solidified lumps during a robot inspection of the containment vessel of Fukushima's wrecked No. 3 reactor. The 3-day survey ended on Saturday.
TEPCO had earlier only made public still images from the probe. The 4-minute video shows black or grey lumps hanging down close to a structure just below the reactor.
The lava-like lumps are piled in layers, a phenomenon unknown before the accident.
TEPCO officials say the debris is probably melted nuclear fuel mixed with broken reactor parts.
In addition to metal scaffolding and other structural components, rocks and sand-like sediment can be seen getting stirred up by the movement of the robot.
The government and TEPCO plan further analysis of the footage in order to determine methods for removing the debris.