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.

That 300-ton 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,

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.