NHK has learned operators of the crippled Fukushima Daiichi nuclear plant believe they may have located molten fuel inside one of the plant's reactors.
If confirmed, this would be the first time since the 2011 nuclear disaster that they have managed to determine the exact location of the debris.
The operator of the plant, Tokyo Electric Power Company, began sending a remote controlled camera into the containment vessel of the Number 2 reactor last week.
On Monday, it transmitted images of a black mass on the floor under the reactor core.
The operator suspects it is fuel debris, a mixture of molten fuel and broken interior parts.
It plans to send in a robot equipped with a radiation-measuring device early next month to get a better idea.
TEPCO plans to hold a news conference on Monday to explain more about the finding.