The moon’s nearside (that is, the side facing Earth) is dark-colored and dominated by ancient lava flows, whereas the farside is more rugged—and NASA researchers now suggest it’s due to a wonky lunar interior. Using data from twin spacecraft named Ebb and Flow, they found a 2–3% difference in the moon mantle’s ability to deform on each side. They say this data could be explained by the nearest hemisphere’s insides being up to 170°C hotter than the farside.