An explosion Tuesday at a hydroelectric plant in northern Italy killed at least three workers and seriously injured five others, a regional fire chief said. At least four other people were missing.

The blast occurred between 100 and 200 feet below the water level at the Bergi plant in the Apennine Mountains south of Bologna. The explosion collapsed two levels of the underground structure, provoked a fire and caused flooding, regional fire chief Francesco Notaro told SKY TG24.

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The explosion happened during maintenance work on the electrical system, blowing out a turbine, Bologna Prefect Atillo Visconti told SKY.

Search efforts were proceeding with care due to the risk of additional collapsing on the 7th and 8th underground levels, while the 9th level was flooded, officials said. Divers were aiding the search.

Video provided by firefighters showed smoke rising from underground at the lakeside powerplant belonging to the Enel power company.

The artificial Suviana Lake was formed by the construction of a dam from 1928-32 and located in a regional park at an altitude of 1,640 feet some 40 miles southwest of Bologna. Enel Green Power said in a statement that there was no damage to the dam.

Premier Giorgia Meloni said she was following the "terrible" news and expressed solidarity with the victims and their families.