The operator of the Fukushima Daiichi nuclear power plant says the groundwater level briefly plummeted near a building that houses one of the crippled reactors.
Tokyo Electric Power Company, or TEPCO, says the fall was observed in a monitoring well about 11 meters southwest of the No.4 reactor building on Wednesday.
The utility says the groundwater level temporarily sank roughly 1 meter below the level of contaminated water inside the reactor building.
The firm says the groundwater rose above the usual level 23 minutes later.
A sharp fall in the groundwater level just outside reactor buildings could cause contaminated water to leak from inside the buildings.
TEPCO says it assessed the density of radioactive substances in the well water on Thursday and has confirmed no leak of contaminated water took place.
TEPCO stopped pumping out water from the well and reported the case to relevant local governments and the Nuclear Regulation Authority.
The company says it is conducting an investigation, suspecting that improvement work on another well 6 meters away may have caused the drop.
The utility publicized the drop on Thursday, one day after the phenomenon was recorded. The firm apologized for the delay in disclosure, saying it initially decided a problem had developed with the well's water gauge as the water level in a nearby well remained unchanged.
The No.4 reactor building experienced a hydrogen explosion, but not a meltdown, during the 2011 accident.