Radioactive water has leaked from a storage tank into the ground at Fukushima plant in Japan.
Tokyo Electric Power Company said the leak of at least 300 tonnes of highly radioactive water was discovered on Monday morning.
The plant, crippled by the earthquake and tsunami in 2011, has seen a series of water leaks and power failures.
The tsunami knocked out cooling systems to the reactors, three of which melted down.
Officials described the leak as a level-one incident - the lowest level - on the International Nuclear and Radiological Event Scale, which measures nuclear events.
However, the BBC reports this is the first time that Japan has declared such an event since the 2011 earthquake and tsunami.
Kyodo news agency said a puddle of the contaminated water was emitting 100 millisieverts an hour of radiation.
TEPCO said on Tuesday that the water probably leaked from a tank after escaping a concrete barrier.
Kyodo reported the remaining water in the tank would be transferred to other containers.
Water is being pumped into the reactors after cooling systems were knocked out by the tsunami.
Hundreds of tanks were built to store the contaminated water. Some of them had experienced similar leaks since 2012, but not on this scale.