A robot swimming in the depths of one of Fukushima’s nuclear reactors may have spotted lumps of molten nuclear fuel inside. If it did, it would be the first robot to successfully locate the radioactive material, as efforts to clean up after the 2011 nuclear accident at the Fukushima Daiichi power plant in Japan continue.

This latest robotic investigator, nicknamed the Little Sunfish, was sent into the Unit 3 reactor for the first time on July 19th. That’s one of the three nuclear reactors that melted down after a massive earthquake and tsunami struck Japan in 2011. Based on earlier surveys, the plant operator, Tokyo Electric Power Company or TEPCO, suspects that the melted fuel in Unit 3 might have burned through the bottom of its container and dropped into what’s called the primary containment vessel. That’s what shields the outside world from the radioactive materials inside.

Powered by five propellers and sporting a camera on its front and back ends, the football-sized robot was remotely operated via a tether attached to its rear. On its first trip, the Little Sunfish successfully navigated underwater. And on its second visit a few days later, the Little Sunfish snapped photos of what look like hardened lumps of lava that may contain melted nuclear fuel. Experts will need to analyze the photos to be sure, but a TEPCO spokesperson told the Japan Times, "There is a high possibility that the solidified objects are mixtures of melted metal and fuel that fell from the vessel.”

Finding this massive source of radiation is among the first challenges TEPCO will need to overcome in order to decommission the plant. Nuclear power plants are fueled by pellets of uranium, packed together inside hollow metal rods “like peas in a pod,” according to the Union of Concerned Scientists. These fuel rods are part of the nuclear reactor’s core, which keeps producing heat even after the reactor shuts down. That’s why it’s so important to keep nuclear reactors cool: if temperatures climb too high, the reactor core can melt into a kind of radioactive lava.

When the tsunami flooded the Fukushima Daiichi plant after the 2011 earthquake, it took out the backup power generators and cooling systems. Over the next three days, the reactors melted down — and since then, the plant operator has been hunting for the molten messes of metal and radioactive fuel left behind.

So far, at least seven of the robots sent to investigate the reactors at Fukushima Daiichi have broken down during their missions. One robot’s camera was fried by high levels of radiation, another got caught on debris and abandoned. The Little Sunfish successfully made the trip not once, but twice into Unit 3. Attempts to remove the melted radioactive fuel probably won’t even start until after 2020, the Associated Press reports — but this small win could be a sign that robots might be able to help the cleanup efforts, after all.