Japanese officials suspect a partial meltdown of fuel rods is to blame for high levels of radioactive contamination in water inside the Fukushima nuclear power plant.
On Monday, work began on removing the water and the plant operator said contamination was again detected in nearby seawater.
For several days, Tokyo Electric Power has known a pool of water inside a turbine building adjoining reactor-2 is highly radioactive. The water is 100,000 times more toxic than water typically found in a reactor core, but how it got that way has been a mystery.
On Monday, Chief Cabinet Secretary Yukio Edano said the government thinks it has the answer.
Mr. Edano says the government believes the water came into contact with partially melted fuel rods and then leaked out of the plant's water system to collect in the basement of a building.
Preparations to remove the water, which exists in varying degrees of radioactivity in three buildings, are underway.
Those preparations became more urgent on Monday when workers discovered the highly radioactive water in a tunnel outside the complex buildings.
Its presence in the tunnel raises the chance that some could seep into the sea, which is only meters away.
While it remains, it limits the amount of time workers can spend inside the building before getting exposed to dangerous levels of radiation.
The plant operator is also hoping to avoid the water leaking into the sea, where it could cause further pollution.
Levels of radioactive Iodine-131 in seawater to the south of the plant are more than 1,000 times normal, in the past few days.
On Monday, Hidehiko Nishiyama, deputy director of the Nuclear and Industrial Safety Agency, provided an update.
Nishiyama says the latest data shows Iodine-131 at 1,150 times the legal limit has been detected one-and-a-half kilometers to the north of the plant.
He says there is no public health risk, at this time.
In the 20 kilometer evacuation zone around the power station, atmospheric radiation levels continued their slow decline on Monday.
Some residents have ventured back into the zone to visit their houses and pick up belongings. The government is appealing to people to keep away, and warns the area remains a risk to human health.
Meanwhile, the area continues to be shaken by aftershocks. A magnitude 6.5 quake jolted hard-hit Miyagi prefecture early Monday morning.