To date, Earth has been the only planet in the Solar System known to demonstrate tectonic activity. Other planets are geologically active, but not much more than that. However, it now looks like our homeworld isn't alone. Smithsonian researchers combing through high-resolution images from NASA's Messenger spacecraft have determined that Mercury is still tectonically active. The team spotted many relatively young fault scarps (surface offsets), which are strong indicators that the minuscule planet's crust is still shrinking as the core cools down. As Gizmodo notes, common wisdom previously dictated that Mercury's interior would have settled down billions of years ago.
Messenger also picked up signs of a magnetic field, which you should see in a planet where at least some of the core is still molten and thus active.
This doesn't meant that Mercury's activity is a carbon copy of what you see on Earth, mind you. There are no tectonic plates shuffling around, so you're not about to see continents or other familiar features. The shrinking crust might be triggering quakes even now, however, and it might only take a seismic probe to confirm that the planet is rumbling.
The scarps could prompt a rethink of planetary development. If Mercury can take so long to completely cool down, what does that say about Earth and other terrestrial planets? While the discovery isn't likely to fundamentally challenge geology, it shows that humans still have a lot to learn about other worlds.