The Wood between the Worlds is a pond-filled forest in The Magician’s Nephew (1955), the sixth book in The Chronicles of Narnia by C. S. Lewis. Each pond is a portal that provides instant transportation to a different world, such as Earth, Narnia or Charn. […]

The salient feature of the wood, other than the trees, is the presence of many pools of water. Initially, the pools appear to be just shallow puddles. However, when someone jumps into one of the pools while wearing another magic ring, the pool of water transports the wearer to a different world. When a world is destroyed by having all life removed (like what happened to Charn in The Magician’s Nephew), the pool dries up.

Source: Wood between the Worlds – Wikipedia