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 of a leak from a tank used to store contaminated water used to cool the reactor cores.
That 300-ton (80,000 gallon) 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.
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.
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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 favorable sampling results.
"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.