So much rain fell that the void – 60 metres below Fort Regent – overflowed four times into an emergency pipe which discharges into the sea near the Elizabeth Marina.
Duncan Berry, assistant director of liquid waste, said: ‘From the beginning of September until 9 October the Cavern has been used six times.
‘Five out of those six times were during Storm Alex, when an unprecedented amount of rainfall fell in a short period.
‘Out of the six times it has filled to its capacity of 25 million litres four times. When the Cavern is full it overflows a small amount of diluted sewage, which goes out to sea.
‘This occurred four times during the storm.’
He added: ‘The ability to discharge excess water into the sea is a safety feature designed to protect inland areas and properties from being flooded with sewage when on the rare occasions surface water overloads the network.’
Last year, the Cavern was taken out of operation while a new automatic cleaning system – aimed at reducing the number of times workers have to enter the structure – was installed.
The work was carried out during the summer, at time of expected reduced rainfall.
However, the Island was hit with weeks of repeated deluges – causing diluted sewage to spill into the sea 40 times.