Eight members of a tour group exploring underground tunnels in Moscow’s sewer system have been found dead after being caught in flash flooding.

The victims, who were touring an underground river known as the Neglinka, are believed to have become trapped by rising water levels, following a downpour on Sunday and were unable to escape to the surface.

The Neglinka was an above-ground river two centuries ago, before being pushed underground in tunnels which doubled as sewers. Some parts have been reconstructed, and other older parts preserved as examples of engineering.

Many large American cockroaches can now be found in the sewers after they were brought to Moscow in the 18th century, according to the city’s government.

Among those who drowned were the tour guide and seven members of his party, which included a minor, local media said.

“The bodies of all participants of the excursion have been found,” investigators said, adding formal identification of the victims was underway.

Moscow Mayor Sergei Sobyanin called the incident a “terrible tragedy” on social media and expressed his condolences.

Several tour operators offer trips into the vast tunnels of the capital’s sewer system, some of which were constructed during the 19th century.

The legality of such trips is disputed, and investigators said on Monday that they had launched a criminal case into an “illegal excursion”.

One of the tour organisers has already been detained, Russia’s Investigative Committee said on Tuesday, adding that another defendant is thought to have fled to the United Arab Emirates.

AFP/ABC