Anyone who knows about the latest incident is being urged to contact Police.
Swansea Council has been forced to close Blackpill Lido for repairs after an overnight attack by vandals left the attraction with a huge cooking oil slick.
The council and the police are using CCTV in the area to try to identify the vandals who carried out the attack.
Robert Francis-Davies, Cabinet Member for investment, regeneration and tourism, said: "It's unacceptable and it's unbelievable. We are in the height of the summer season and on the sunny days we've been having recently the Lido has been packed.
"After this latest incident we've got no option but to close the Lido for urgent repairs. We know how much people love the Lido and we're doing all we can to get it fixed as quickly as possible. But it will not be open today (Thursday) and we'll have a better idea of when it can re-open after we've carried out a full assessment.
"We'd urge anyone who knows anything about this latest incident to contact South Wales Police."
It's understood vandals turned up last night with two large drums of cooking oil and poured the contents across the lido, spreading like an oil slick across the surface of the water.
The council cannot risk damaging filter systems on the Lido which means it will take time to clear the mess, check filters and other systems and replace contaminated water with fresh water.
Cllr Francis-Davies added: "We're very sorry that children and their parents are going to be disappointed that one of their favourite free attractions on the prom won't be open.
"Clearing up the mess will take time but we will do all we can to get it back up and running as soon as possible."
Following a vandalism attack on the lido last month, it had to be closed for a number of days to make sure that all the glass found there had been removed before it re-opened.