Highly radioactive water flowed into the ground from a storage tank at the crippled Fukushima nuclear power plant on Wednesday, in what officials said was the largest contamination leak in roughly six months.

The water, reading high levels of radiation, overflowed from a large storage tank after a valve that had been mistakenly left open allowed excessive amounts of contaminated water into the tank.

The overflow was discovered Wednesday night, but before the valve was closed the tank is estimated to have spilled 100 metric tons of water containing 230 million becquerels per liter of “beta-emitting radioactive isotopes, including strontium 90.”