Perhaps the most critical piece of evidence that tells us humans have been to the moon and back is the fact that we brought back samples of the moon to Earth, and they are very clearly unlike what we can find on our own planet. Throughout six different Apollo missions, NASA astronauts brought back about 842 pounds of moon rocks for scientists to study—the oldest of which are 4.5 billion years old, or 200 million years older than Earth’s oldest rocks. Analyses and experiments have definitely shown over and over that these aren’t geological materials you can just collect on the fly from Earth. They’re almost completely void of volatiles (chemical elements and compounds with low boiling points found in terrestrial crust and atmosphere, like nitrogen, carbon dioxide, water, etc.), as well as hydrated materials. These rocks simply do not exist on the modern-day Earth.