When Apollo 13 was being planned, scientists were specifically looking to the answers raised by the Apollo 11 and Apollo 12 missions. One strange occurrence that they were attempting to answer was just why the moon rang like a bell for an hour. During the Apollo 12 mission, astronauts took a 2.5 ton piece of metal and crashed it into the surface of the moon. This led to a strange bell like sound to reverberate for over an hour, leading scientists to believe that the Moon’s structure is different from the Earth.
Apollo 13 did a similar experiment with larger and heavier items and encountered the same result. Strangely, NASA is unable to explain why the moon does this with the only theory, which is highly unlikely, being that it is hollow!