A damaged reactor at the Fukushima Daiichi nuclear power plant suddenly began releasing steam again, but it did not appear to be a result of renewed nuclear reactions — a worst-case situation that could lead to a large new release of radioactive materials.

Still, the steam was another indicator of the fragile state of the plant more than two years after the worst nuclear disaster after the accident at Chernobyl. The latest news adds to revelations of a slew of other problems at the plant, including indications that radioactive water may have been leaking into the ocean since the disaster.

It is still unsure what caused the release of steam. In fact, there are many phenomena that could have produced the steam such as rainwater seeping into the reactor’s damaged containment vessel and turning into vapor, or live electrical cable falling into a puddle.

More than 100,000 people fled their homes after the meltdowns at Fukushima. While many areas not immediately surrounding the plant are beginning to be repopulated, the news of the residents’ return has been overshadowed by the continued problems at the plant.