Tokyo Electric Power Co. said Saturday that the trench problem at the crippled Fukushima No. 1 nuclear plant has cropped up again and is sending highly radioactive water into the sea.

The water in the underground passage, which runs under the turbine building of reactor 2, contains 2.35 billion becquerels of cesium per liter, roughly the same as that measured right after the crisis began in spring 2011.

The latest sample, taken Friday from a trench, contained 750 million becquerels of cesium-134, 1.6 billion becquerels of cesium-137 and 750 million becquerels of other radioactive substances, the utility said.

A sample from April 2011 contained 1.8 billion becquerels of both cesium-134 and cesium-137 per liter. Cesium has a half-life of about 30 years.

The trench is believed to be the source of the groundwater problem that’s been baffling Tepco’s experts for months. Their current theory is that the highly radioactive water found and left in the trench in 2011 is now leaking directly into the groundwater, which is seeping into the sea.

Tepco finally admitted Monday that contaminated water was getting into the Pacific. The admission came after the Nuclear Regulation Authority pointed out that highly radioactive water was “strongly suspected” to be seeping into the ground under the site and making its way to the sea.

The utility hopes to halt the problem by building a wall out of liquid glass between the reactors and the sea and removing the contaminated water from the underground passage.