Newtonian gravity couldn’t explain this, but Einstein realized that his budding new Theory of General Relativity could. Einstein was able to accurately predict Mercury’s orbit based on the Sun’s effect on space-time.
2. Mercury is geologically active. The tiny planet is contracting, and new faults are appearing on its surface. This means that Mercury is the only geologically active planet besides Earth in our Solar System that we know of.
3. Mercury’s hollows: A big ol’ geological mystery. Mysterious shallow depressions peppering Mercury’s impact craters are very young and could be forming today... but no one knows why, or how.
4. Mercury is the only other rocky planet in the Solar System with a magnetic field. Although Mercury’s magnetic field is just one percent as strong as Earth’s, it's evidence that the small planet still has a molten core.