In my Leahy 624, there is a slot above all of the hatching trays that a very big metal pan would rest on, full of water. I don't think that the pan was more than 1 1/2 inches tall. After you filled it with water, then placed it where the fan was blowing air across the water, that would create a Humidifier effect, causing the humidity to rise and stay pretty high. Unfortunately, most people that had the metal pan for their incubator probably lost it, or it got rusted, and started leaking. Cheap Metal+water= rusty, hole filled water pan! I don't assume that the water pans lasted for very long. I would love to get my hands on one, or even a replica of the original pan.