(Bloomberg) -- A deadly explosion at an Enel SpA hydropower facility in northern Italy that left at least three people dead occurred as works to renew facilities at the 1970s-era plant were concluding. 

The accident took place some 40 meters (131 feet) underground Tuesday afternoon amid maintenance to upgrade the Enel Green Power plant in Bargi, near the city of Bologna. A subsequent explosion led to the collapse of a building structure, with a water pipeline breaking and flooding the site, Italian media reported.

A spokesperson for Italy’s national fire department confirmed three deaths at the facility, speaking to Sky TG24 television. Four workers remain missing, according to the department. 

Bologna-based prosecutors said they’ll open an investigation into responsibility for the accident, with a possible focus on contractors hired for the renewal of the plant’s turbines. Enel Green Power said in a statement that it’s cooperating with authorities.

Enel Green Power also said Wednesday that contractors included units of Siemens AG, ABB Ltd. and Voith GmbH & Co KGaA. 

The plant, which started up in 1975, can pump water into a nearby reservoir to balance the grid at times of oversupply. It’s the largest such facility in the Emilia Romagna region, and can produce 330 megawatts of power, enough to supply about 600,000 average European households.

Italy has a history of harnessing hydropower dating back to the 19th century. The country had around 4,800 hydro plants in 2022, according to data compiled by electricity transmission operator Terna SpA.

It may prove difficult to quantify the potential economic impact of the accident for Rome-based Enel, Equita SIM wrote in a note on Wednesday. “Assuming a total production loss of around 354 GWh in one year, the impact could be approximately €20 million ($21.7 million) in terms of Ebitda,” the broker said, adding that that does not include insurance reimbursements or the “operational continuity of one of the two units” at the facility.

(Updates with missing workers in third paragraph, photo after fifth.)

©2024 Bloomberg L.P.