Archived Kimimaru4000 video from YouTube. Original description mostly intact:

After much experimentation, I finally did it! This video shows how to get into Sirena Beach early on in the game and without Yoshi. The building has a small ledge ringed around the inside of it that can be stood on, as well as some solid ground very close to the pipe in the middle of the building. By glitching into the building with the right height and doing a Ground Pound, I successfully manage to make it onto the ledge. After that, all I do is make it onto the other piece of solid ground and Spin Jump into the loading zone for Sirena Beach!

Note that the loading zone is not there when you first enter Delfino Plaza; you must first defeat the Polluted Piranha Plant and uncover the Grand Pianta Statue.

Also note that after a specific point in the game (I'm not sure which one, but probably after the cutscene in which you gain access to Pinna Park), the building becomes completely solid and you become unable to glitch through it. As a result, this must be done relatively early on in the game.

Also, sorry for the bad quality; I'm at college right now so I don't have my camcorder.

Without further ado, enjoy!

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