Chaos Pool

The Chaos Pool was the place in which the Key to Time was forged, and where it could be safely destroyed without affecting the whole of the universe. It was located on the Planet Chaos, though the Doctor, Zara (with Amy concealed 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, to destroy it. The pool vanished, making Romana, Zara and Amy think the Doctor 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)