Collect the Caps... - Super Mario 64 Walkthrough
Dire, Dire Docks: Star #6
In the part of the level where the submarine was you'll find a Vanish Cap and Metal Cap. Only the former is actually needed, but you're supposed to use both to get down to the star that's located in the cage at the bottom of the water.
Draining the moat to get the Vanish Cap: http://youtu.be/3xec_Iph1Es
Getting the Metal Cap: http://youtu.be/7msP_QHrp00
スーパーマリオ64 攻略 - ウォーターランド - ぼうしがそろったら
Super Mario 64 Walkthrough Playlist: https://www.youtube.com/playlist?list=PLWdUn5TiM4AlbtvpLuO2bRnvnedFLY43g
Want to buy this game?: https://club.nintendo.com/rewards-details/a/35506.do
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