Captured comet?
Old spaceship parked in orbit millions of years ago. It’s hollow.
Mars has craters, but the distribution of those craters shows that many of them on one side of the planet were formed in a single impact event — either a grapeshot-style with a single bolide breaking up from Mars’ gravity and smashing in many places more or less simultaneously, or more likely, one large impact which threw up ejecta. Could have been an earlier moon of Mars that did it, but Mars has more potential exposure to large asteroids than we do. The two moons probably arrived at the same time, as the odds of capture are much better when three bodies are involved, rather than two.