How do I bury myself in the sand?

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I am really confused about what I am supposed to do after I get the disc from the guy in the castle, and I heard something about burying myself in sand to fight someone. Where is this sand, and what do I do afterwards?
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I Buried Myself Alive on the Beach and It Went Incredibly Wrong (Buried Alive Challenge)
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Images: thiago japyassu, Kampus Production, Tima Miroshnichenko, Kampus Production