Once on Soggy Island, head to the monument and talk to the security guard. Now, head up the mountain to talk to the scientist and pass his quiz (None, 365, 20). Return to the monument and complete the pipe puzzle to get inside. Then, jump on one of the pedestals, it will light all of the pedestals up. The one you are on will be number 6, so now you need to jump on them in descending order.
Here is a video guide with English commentary!