A Waste Disposal Unit is featured prominently in the room Waste Disposal, in the Phazon Mines in Metroid Prime. The name suggests it is a garbage disposal, though it appears to be filled with water. Samus navigates these tunnels, using Bombs to shoot up onto ledges in the water to get between the Main Quarry and Ore Processing. When Samus, in Morph Ball mode approaches the small hatches at the entrances of each side of the Disposal Unit, she is sucked in.
The room is explored on Samus' return trip up, when she has acquired the Power Bombs. It can be used again in other trips as well.
"Caution. Waste Disposal Unit will draw small objects inside automatically."