The operator of the crippled nuclear plant in Fukushima has released photos taken inside a reactor building.
The utility believes it may have located molten fuel within the plant.
NHK World's science correspondent Kenichiro Okamoto has been following events at Fukushima Daiichi since the disaster. He explains what the news could mean.
Okamoto: The discovery is potentially a big deal. If it turns out that officials have photographed molten fuel, it would be an important step in the decommissioning process.
They've spent years looking for the fuel and if they've found it, they can start to come up with a plan for the plant.
Here's what we already know. There's probably a lot of molten fuel at the bottom of the reactor. Images from last year told us that.
But we didn't know how much fuel, if any, was further down in the reactor's containment vessel. The point of the vessel is to stop radiative materials from leaking out.
Monday's probe showed deposits on a metal grate within the containment vessel under the reactor core. If that turns out to be molten fuel, it would be the first they've found since the 2011 disaster.
It would mean fuel mixed with debris penetrated the reactor and fell into the containment vessel underneath.
The cleanup process could be sped up, depending on how much fuel has leaked out. At the moment, we still don't know.
The government and TEPCO plan to send in a robot equipped with a radiation-measuring device early next month to make an assessment.
Here's the schedule the government wants to follow.
It intends to draft a plan by this summer on how to remove the debris from all 3 reactors.
It wants to finalize the details -- including a decision on which reactor to work on first -- by the first half of fiscal 2018. And it wants to begin actually removing the debris by 2021.
It says the whole process could take up to 40 years.