The Mermaid's Singing Rock (人魚の歌岩, Ningyo no Uta Iwa) is an event in Tomba!.
Get to the Mermaid Singing Rock.
You should clear the previous event to complete this event.
After you get the fuel from the GoGoCar man, go to the Village of All Beginnings.
Then go to the "garage" from the first house, you will find a GoGoCar, use the Fuel bar and ride it to the finish line. Walk through the beach and you will find a stone arch. Walk to the arch, you will find the mermaid in a rock and the event will be cleared.