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. 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. 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.