Staff at a public swimming pool arrived at work to find the water had gone missing.
Leisure centre staff at Aquarena, in Brighton Road, Worthing, turned up to discover a million litres of water had leaked away overnight.
On opening the door to the boiler room underneath, they discovered it awash with the chlorinated water.
Firefighters were called in yesterday morning and spent most of the day pumping the water out of the boiler room into the sea. Permission was granted by the Environment Agency.
Sub Officer Darren Wickings said: "When staff got to work they noticed a significant amount of water missing from the pool and didn't know where it had gone until they opened the door to the boiler room.
"There is a very serious leak but it's slow."
Staff were unsure how the leak happened. The plant room, which is in the basement, was too submerged for staff to discover where the water was coming from.
The boiler, water treatment and water filter machinery are housed in the flooded room but the amount of damage caused has not been calculated.
John Thorpe, assistant director of community services at Worthing Borough Council, said: "The plant room was flooded when staff arrived at 8am.
"The fire services were still pumping it out. Until they've finished completely we cannot ascertain what the cause of the problem is but it's likely to be a failure in the pipe work.
"The problem is, with it being the Christmas period, it might be some time before people can come and inspect the damage to machinery."
The pool was closed for maintenance work and due to reopen on December 27.
Council staff expect the date will be put back for further repair work.
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