Goop Generator

From the Super Mario Wiki

A Goop Generator is a puddle of Goop that cannot be washed away in Super Mario Sunshine. They are always located in the middle of a large area of Goop. If Mario attempts to wash away the Goop, the Goop Generator will create more Goop. If Mario sprays the Goop Generator with F.L.U.D.D., a Piranha Plant in the Generator will come out and attack. Mario must defeat the Goopy Piranha Plant to make the Goop Generator and its surrounding Goop disappear and let the object it's under rise up again.