In all our searching of the universe, we’ve never seen another moon like ours. It's big, it's weird, it's played a huge role in shaping our planet. But how did we get it? Every possible story points to cataclysm. This is the second episode of our new four-part series, Lost Worlds. For more, go to http://vox.com/unexplainable It’s a great place to view show transcripts and read more about the topics on our show. Also, email us! unexplainable@vox.com We read every email. Support Unexplainable by making a financial contribution to Vox! bit.ly/givepodcasts Learn more about your ad choices. Visit podcastchoices.com/adchoices