A guided tour through Moscow’s sewer system went horribly wrong on Sunday when a downpour flooded the underground tunnels while the tour group was inside.

Now, a frantic search is underway for at least eight missing members of the trip. Three teenagers have been found dead so far, one 17-year-old and two 15-year-olds, according to state-run media.

“The body of an unidentified woman has been found in the Moskva River, making her the third person to die after a tour of the underground sewers,” an emergency services source told the TASS news agency.

Eight people were known to be on the disastrous tour, but officials fear more could be missing. Two dozen people had reportedly signed up to take the tour, but it’s unclear exactly how many ended up going.

Sunday’s trip apparently went haywire when heavy rainfall caused the sewer’s water levels to quickly rise, and the group was unable to escape the tunnels. Tour guide Konstantin Filipov, 31, is among those missing. In his last text message, he pleaded for an early exit of the tour.

“Fuck, it’s raining. Can I go out at Trubnaya?” his message said.

Daniil Davydov, an urban historian from Moscow, told the RIA news agency that there were escape shelters in the tunnel for emergencies like this. Unfortunately, nobody was found in them.

“I hoped that maybe I would still be able to find some survivors there. There are two shelter points, but there was no one there,” Davydov was quoted as saying.

The tour organizer is under investigation by local law enforcement, Russian media reported.