An explosion at a hydroelectric power plant in northern Italy has left four dead and several others missing.
An underwater explosion occurred at the plant in Lake Suviana, 70 km (43 mi) from the city of Bologna.
The affected area is located 30m (100 ft) below the surface of the lake.
The mayor of Camugnano, a nearby town, told Anza news agency that the fire was believed to have started during maintenance work.
Marco Masinara said rescue workers were having trouble getting access and that the crash was “serious.”
Three of the four who died in the accident were aged 35 and 73.
Bologna's presiding officer, Attilio Visconti, was eight stories below the surface when a turbine exploded causing the fire to flood the floor below.
A diver involved in the search and rescue operation told Italian newspaper La Repubblica that they were able to reach the seventh floor below the surface, but could not reach the three people trapped further down.
Several people are known to have suffered severe burns following the explosion at around 15:00 (13:00 GMT). They were airlifted to nearby hospitals.
Italian media reports that two men have been admitted to intensive care.
Bologna Mayor Matteo Lebor, who visited the scene on Tuesday evening, said conditions for search teams dozens of meters underground were “really complicated”. He said it was one of the worst labor tragedies in Italy's recent history.
The city's chief prosecutor, Giuseppe Amato, told Italian media that an investigation would begin once the plant was safe and the missing people found.
“We won't stop until we find them,” said Luca Cari, representing the firefighters.
The cause of the accident has yet to be determined, but what happened has fueled concerns expressed by unions about workplace safety in Italy.
The country's two largest unions had already called for a four-hour nationwide strike on Thursday to protest the issue.
Regional Fire Chief Francesco Notaro said firefighters and police “are all trying to protect the facility.”
Lake Suviana is an artificial lake located in the Apennine mountain range near Bologna. The lake was created following the construction of a dam in 1932.
Italian Prime Minister Giorgia Meloni X earlier on Twitter thanked rescue services and said she was “concerned” following news of the explosion.
The power plant is owned by Italian energy company Enel Green Power.
There were no signs of damage to the dam, but the power plant had stopped production, the company said.