TOKYO (AP) -- 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 toward the Pacific.
Now, 2 1/2 years later, experts fear it is about to reach the ocean 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 that stores contaminated water used to cool the reactor cores. That 300-ton (80,000-gallon) leak is the fifth and most serious from a tank since the March 2011 disaster, when three of the plant's reactors melted down after a huge earthquake and tsunami knocked out the plant's power and cooling functions.
But 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 farther into the sea. Most fishing in the area is already banned, but fishermen in nearby Iwaki City had been hoping to resume test catches next month following favorable 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 groundwater.
About 1,000 tons of underground water from the mountains flows into the plant compound each day, of which 400 tons seep into the reactor and turbine basements and get contaminated. The remaining 600 avoids that area, but at least half of it is believed to eventually come in contact with contamination elsewhere before entering the sea, according to an estimate by Japan's Agency for Natural Resources and Energy.
Scientists, pointing to stubbornly high radioactive cesium 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 out of the reactor and turbine 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.
The turbine buildings at the Fukushima Dai-ichi plant are about 150 meters (500 feet) from the ocean. According to a Japan Atomic Energy Agency document, the contaminated underground water is spreading toward the sea at a rate of about 4 meters (13 feet) a month.
At that rate, "the water from that area is just about to reach the coast," if it hasn't already, said Atsunao Marui, an underground water expert at the National Institute of Advanced Industrial Science and Technology who is on a government committee studying the contaminated water problem. "We must contain the problem as quickly as possible."
TEPCO, nationalized and burdened with the astronomical cleanup costs, has been criticized for repeatedly lagging in attempts to tackle leakage problems. As a precautionary step, it has created chemical blockades in the ground along the coast to stop any possible leaks, but experts question their effectiveness. After a nearly two-year delay, construction of an offshore steel wall designed to contain contaminated water has begun.
The utility has also proposed building frozen walls -- upside down comb-shaped sticks that refrigerate surrounding soil -- into the ground around the reactor areas, but that still has to be tested and won't be ready until 2015 if proved successful.