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.