How do Mario and Lu Yiji rpg joke about their final destination?

On the beach to the east of Doudou King City, where you walked from the bottom of the sea to the land for the first time in the plot, there is a black stone, which is broken with the highest hammer (the hammer can go to the blacksmith's cave to upgrade). You will see a hole in the ground, where you can drill the ground, and you will see ... and then you can go surfing (the whirlpool on the sea can enter and reach some places that you can't usually go).