It’s been almost six years since a tsunami damaged the Fukushima Daiichi nuclear power plant on Japan’s northeast coast, causing meltdowns of three nuclear reactors. And scientists are continually learning more about managing the disaster. New readings taken inside reactor No 2 are the highest recorded since the accident, reports Justin McCurry at The Guardian.

According to a report by The Japan Times, Tokyo Electric Power Company (Tepco), the owner of the power plant spearheading efforts to decommission the radioactive site, used a camera on a telescopic arm to look inside reactor No 2 last week. They found that the material housed inside the pressure vessel—the metal capsule used to hold the nuclear material within the containment unit—had likely melted through the receptacle's bottom and created a three-foot hole in the grating that lies underneath. Images also show black debris that may be some of the melted nuclear fuel, which would be the first material located by Tepco since the disaster. It’s believed the fuel melted through the pressure vessels in the other two reactors as well. The material, however, remains safely within the outer containment vessel and only poses a risk within that protective barrier.

Tepco, however, is not willing to confirm the find just yet. “It may have been caused by nuclear fuel that would have melted and made a hole in the vessel, but it is only a hypothesis at this stage,” Tepco spokesman Tatsuhiro Yamagishi tells Agence France-Presse. “We believe the captured images offer very useful information, but we still need to investigate given that it is very difficult to assume the actual condition inside.”

But exploring further may prove difficult. Examining the electronic noise caused by radiation in the images taken near the pressure vessel, Tepco analysts determined that the area is contaminated by 530 sieverts of radiation per hour. The previous high in the reactor was 73 sieverts recorded in 2012, reports The Japan Times. Luckily, there is no indication that the radiation is leaking outside the reactor.

One sievert—the international measurement of radiation exposure—is enough to cause radiation sickness, infertility and cataracts. Exposure to 10 sieverts will lead to person’s death within weeks, reports McCurry. Tepco says that their estimate has a margin of error of 30 percent, but even then the radiation levels are off the charts. This does not, however, necessarily mean that radiation levels are increasing, notes Safecast, an organization devoted to citizen science. Radiation has not previously been measured in this location. They explain:

It must be stressed that radiation in this area has not been measured before, and it was expected to be extremely high. While 530 Sv/hr is the highest measured so far at Fukushima Daiichi, it does not mean that levels there are rising, but that a previously unmeasurable high-radiation area has finally been measured. Similar remote investigations are being planned for Daiichi Units 1 and 3. We should not be surprised if even higher radiation levels are found there, but only actual measurements will tell.

These high radiation levels, however, complicate Tepco’s plans to continue exploring more of the containment vessel in the coming weeks, reports The Japan Times. The hole in the grating means that operators will have to find another route for the remote controlled vehicle they planned to use. The high radiation level could also cause problems since the remote vehicle is designed to absorb 1,000 sieverts of radiation. If the level really is 530 sieverts, the little robot only has two hours to explore before it is disabled, not 10 hours as previously calculated from earlier radiation readings. In an earlier article in The Guardian, McCurry reports that three previous attempts to place a robot inside reactor No 2 failed when radiation fried the gadgets.

Getting basic data about where the nuclear fuel is located, however, is critical to beginning the 40-year decommissioning process in earnest. “Confirming the conditions inside the reactor is a first step toward decommissioning,” Hiroshige Seko Minister of Economy, Trade and Industry said in a news conference. “While difficult tasks and unexpected matters may arise, we will mobilize all of Japan’s technological capabilities to steadily implement decommissioning work and rebuild Fukushima.”

According to The Japan Times, even if further investigation of reactor No 2 is stalled, Tepco still has plans to send a robot into reactor number No 1 in March to examine water that has accumulated in the basement of the reactor.

McCurry reports that in December, the Japanese government revised a 2013 estimate of the costs of decommissioning the plant to roughly $190 billion, double its initial estimate.

Update 2/07/2017: The headline and story has been updated to stress that radiation levels within the nuclear reactor are not necessarily rising, but the latest measurements are the highest values yet recorded.