The Truth Pond is surrounded by a circle of trees just far enough apart that their thick branches touch. The water in the pond is crystal clear and still as glass.
The Truth Pond has the power to break unwanted enchantments, reverting transformations to their true forms. A side effect is that anyone who bathes in the Truth Pond is compelled ever afterward to tell the truth.
During their journey to Oz, the Shaggy Man and Button-Bright had their heads transformed into those of a donkey and a fox. A dip in the Truth Pond returned their heads to normal. Later, after arrving at the Emerald City, the Shaggy Man had to admit that he had stolen the Love Magnet. (The Road to Oz)
The Truth Pond also appears or is mentioned in The Forbidden Fountain of Oz, The Law of Oz, and "The Final Fate of the Frogman".