Secret of the Aquatic Plant (スイ草のヒミツ, Suisou no Himitsu, lit. Secret of the Water Plants) is an event in Tomba! 2: The Evil Swine Return. This event is similar to the Pump Rocks event in the first game.
Break all the Water Plants in the game in order to release the mermaid from her prison.
Talk to the mermaid trapped inside a giant Water Plant in the room above the entrance to Water Temple via the Circus Village. The event is now activated.
After all of the Water Plants has been broken, go back to the room and talk to the mermaid to clear the event.