Four scuba divers died while performing maintenance in an aqueduct that supplies Mexico City, authorities reported Sunday.
The Security Agency of Mexico State, which border Mexico City, said the men were experienced divers maintaining a huge underground line just west of the capital.
Police divers recovered the bodies and state prosecutors said they were investigating the cause of the accident.
Flow through the aqueduct had been stopped prior to maintenance, but the tunnel was still full of water.
The agency said the men were employed by a private company contracted by the National Water Commission. They entered the aqueduct late Friday to clean and inspect it but failed to return to an access well.
Mexico City relies on huge tunnels to pump clean water in and sewage out of its closed mountain valley.
The size, depth and constant use of the aqueducts pose maintenance problems, sometimes requiring the use of boats or teams of divers.
A diver died while cleaning storm drains in Mexico City in 1994.