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 dissapear.