This is a stone that was thrown into Hellfire seventy years ago and it was falling into Hellfire until it reached its bottom

The Prophet (may Allah's peace and blessings be upon him) heard a disturbing sound like that of a falling object. He asked the Companions who were with him about that sound, and they said: Allah and His Messenger know best. He (may Allah's peace and blessings be upon him) said to them: This sound that you heard is a stone that was thrown from the edge of Hellfire seventy years ago, and it kept descending and going down into it until it reached its bottom now when you heard the sound.