Fresh details emerged Wednesday about the volcanic eruption at Mount Moto-Shirane in Gunma Prefecture and how it caught the Meteorological Agency and volcanic experts unaware, prompting the agency to release a delayed volcanic warning.

Commenting on Tuesday morning's sudden eruption, Makoto Saito, director of the agency's volcanology division, told a news conference later in the day that because there had been "no observational data that suggested signs of volcanic activity before the eruption," raising the volcanic alert level beforehand would have been near impossible.

According to the agency, there had been no volcanic activity at the Kagamiike crater, the apparent site of the eruption, for about 3,000 years until Tuesday. Because of this long period of dormancy, it had not been part of the agency's 24/7 volcano-monitoring program.