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The Messenger spacecraft recently flew past Mercury and took photographs of parts of that planet that we've never before seen. The photo above shows mysterious features stretching hundreds of miles across Caloris Basin. They might be remnants of the impact that created the crater at the center of the features, or their placement might just be coincidental. We'll have to continue studying this strange world to know how they formed.
I suggest that the big hole at the center isn't a crater at all but a nest, and the grooves were formed by whatever hatched in the nest as they crawled away shortly before they took flight across the Solar System, threatening life on the planets (for example, they devoured the civilization on Saturn's moon, Iapetus). Comets? Nonsense! They're Mecurians!