The cause of the water leak in the St Venera tunnel remains unclear, though sources told The Sunday Times it was unlikely to involve potable water.

The St Venera tunnels built in 1992 pass beneath Cannon Road at the intersection with Parish Street, alongside the St Venera Parish Church, St Joseph High Road and the gardens of Apap Institute.

The first part of the tunnels from the Marsa side passes beneath some fields but the rest of the section passes underneath buildings.

The area may contain some old cisterns or underground water galleries that may have been linked to the aqueduct that passes along St Joseph High Road.

"It is a possibility that there may be a gallery that floods whenever it rains, or possibly a cracked reservoir," the source said.

A massive leak, which saw water gushing down from the tunnel's ceiling, prompted the temporary closure of the northbound lane last week until the authorities determined whether the situation posed a danger to motorists.

The Water Services Corporation was conducting tests in the area to determine whether there were any leaks in its water and drainage pipes.

The corporation's chief executive would not comment on the matter when contacted yesterday, insisting the WSC would make an official statement when it was certain of its findings after all tests were concluded. However, the foreman who was in charge of the excavation works when the tunnels were constructed has expressed concern that a sewage leak that existed when the tunnels were built may have remained unresolved.

Brian Blackburn, quoted in yesterday's The Times, said a borehole dug up to test the stability of the rock had hit a cave where drainage had been collecting. The drainage gushed out of the cave when the borehole was hit during the digging of the tunnel, he added.

ksansone@timesofmalta.com