Around 120 tons of contaminated water with an estimated 710 billion becquerels of radioactivity has probably leaked into the ground under the Fukushima No. 1 power plant, Tokyo Electric Power Co. revealed Saturday.

"It is the largest amount of radioactive substances that has been leaked" since the crippled facility's cold shutdown was declared in December 2011, Tepco official Masayuki Ono said.

The utility, which announced the leak overnight, said Saturday morning that the water escaped from one of seven underground reservoir tanks at the No. 1 plant and that the remainder — an enormous 13,000 tons — is being pumped to other tanks nearby.