Pluto’s vast, smooth yet oddly textured heart-shaped terrain called Sputnik Planum was a glorious surprise to the New Horizons team, because nobody had ever seen anything like it before. It pointed to a geologically active body, rather than the ancient dead lump everyone was expecting, and the mountains adjacent to it looked straight out of the Rockies, except blockier and less organized, complete with what look like glacial flows.
New Horizons still has the majority of its data queued up for transmission, but already they have enough data to start understanding why this terrain looks the way it does: Sputnik Planum is a gigantic impact basin. Sometime in the geologically recent past, something vast smacked into Pluto, carving out this area and liquifying it and causing nitrogen to spurt out from under the dark substrate that forms the bulk of Pluto and flood the plain. It has yet to fully resolidify, and so the material is more like a nitrogen slush. Nitrogen freezes at the temperatures found on Pluto, but frozen nitrogen is a very weak, smooshy material. Any heat at all could easily produce the bubbly surface in it. Meanwhile, the chaotic mountains around Sputnik Planum were a surprise as well, because nitrogen ice should not be able to support itself to those heights. So it turns out the mountains are water ice, and water ice is less dense than nitrogen ice. The mountains are basically gigantic icebergs floating on top of the nitrogen, shattered and kicked up by the impact that formed Sputnik Planum and huddled around its margins.
There are more mysteries, and far more data still to come; Christmas has come for the New Horizons team, and will keep on coming for a long while yet!