Chaos Pool

The Chaos Pool
The Chaos Pool was the pace in which The Key to Time was forged, and where it can bbe safely destroyed without affecting the whole of the universe. It was located on the Planet Chaos, though The Doctor, Zara (with Amy conceilled within her satchel), Astra, Captain Pargrave, and Lydel had to search for it on the planet's surface. When the key was complete, The Doctor took it with The Grace inside, who were still waiting for the Key to be released to them, and jumped into the Chaos Pool for it to be destroyed. This was the case and the pool vanished, making Romana, Zara and Amy think he was dead, but he was actually stuck on a ledge in the pit the pool left behind. (BFA:The Key 2 Time - The Chaos Pool)