Deep beneath Fukushima's crippled nuclear power station a massive underground reservoir of contaminated water that began spilling from the plant's reactors after the 2011 earthquake and tsunami has been creeping slowly towards the sea.
Now, two-and-a-half years later, experts fear it is about to reach the Pacific and greatly worsen what is fast becoming a new crisis at Fukushima: the inability to contain vast quantities of radioactive water.
The looming crisis is potentially far greater than the discovery earlier this week from a tank used to store contaminated water used to cool the reactor cores. That 300-ton leak is the fifth and most serious since the disaster of March 2011, when three of the plant's reactors melted down after a huge earthquake and tsunami knocked out the plant's power and cooling functions.
Experts believe the underground seepage from the reactor and turbine building area is much bigger and possibly more radioactive, confronting the plant's operator, Tokyo Electric Power Co, with an invisible, chronic problem and few viable solutions.
Many also believe it is another example of how Tepco has repeatedly failed to acknowledge problems that it could almost certainly have foreseen - and taken action to mitigate before they got out of control.
It remains unclear what the impact of the contamination on the environment will be because the radioactivity will be diluted as it spreads further into the sea. Most fishing in the area is already banned, but fishermen in nearby Iwaki City were hoping to resume test catches next month following favourable sampling results. Those plans have been scrapped after news of the latest tank leak.
"Nobody knows when this is going to end," said Masakazu Yabuki, a veteran fisherman in Iwaki, just south of the plant where scientists say contaminants are carried by the current. "We've suspected (leaks into the ocean) from the beginning ... Tepco is making it very difficult for us to trust them."
To keep the melted nuclear fuel from overheating, Tepco has rigged a makeshift system of pipes and hoses to funnel water into the broken reactors. The radioactive water is then treated and stored in the aboveground tanks that have now developed leaks. But far more leaks into the reactor basements during the cooling process - then through cracks into the surrounding earth and ground water.
Scientists, pointing to stubbornly high radioactive caesium levels in bottom-dwelling fish since the disaster, had for some time suspected the plant was leaking radioactive water into the ocean. Tepco repeatedly denied that until last month, when it acknowledged contaminated water has been leaking into the ocean from early in the crisis. Even so, the company insists the seepage is coming from part of a network of maintenance tunnels, called trenches, near the coast, rather than underground water coming from the reactor area.
"So far, we don't have convincing data that confirm a leak from the turbine buildings. But we are open to consider any possible path of contamination," said Tepco spokesman Yoshimi Hitosugi.