Around four billion years ago, earth gained its moon, but it came at a great cost. The leading theory is that another planet we now call Theia slammed into the earth, and the debris that this collision spewed around in space eventually amassed into what is now our moon. We know the earth survived this collision, but what happened to Theia? Some of it ended up in the moon, of course, but that's only a fraction. Some of it was expelled into space. But anomalies in our measurements of the earth and its behavior indicate that there are masses underneath the earth's surface that don't make sense. It could be that large parts of Theia ended up melded with the earth's mantle, and have been with us all along. Hank Green explains what gave us that idea in this episode of Sci Show.