Waterfall Basin Seal
From the Super Mario Wiki
The Waterfall Basin Seal is a key found in Wario: Master of Disguise. It is one of four keys found in the Ancient Waterworks, the others being the Floodgate Seal, Torrent Seal, and Cataract Seal. The purple treasure chest containing it is in a room on the bottom left corner of the ruins, and is accessed by using Cosmic Wario's lasers to make temporary platforms appear. This room is the same as the one where the Floodgate Seal is found, but the section of the room where the Waterfall Basin Seal is located is accessed differently. The key is used to open a locked door with a picture of the key on it leading to the bottom section the central area with the waterfall, which contains the door to the boss's room.
Of the four keys found in the Ancient Waterworks, the Waterfall Basin Seal is the only one that does not appear in Special Episode Part 3.