Phil &#x27;Bad Astronomer&#x27; Plait says the mountains are made of water-ice, the plains of nitrogen-ice [1]: &quot;Pluto is so cold water ice is hard as rock, and can form mountains thousands of meters tall. The bright plain was probably lower elevation terrain which filled with nitrogen, which is much less rigid than water at these temperatures. It can flow like glaciers do on Earth. So what you’re seeing there is a shoreline! It really looks like one, too, with the nitrogen ice meeting the highlands at a “constant sea level”.&quot;<p>[1] <a href="http:&#x2F;&#x2F;www.slate.com&#x2F;blogs&#x2F;bad_astronomy&#x2F;2015&#x2F;12&#x2F;05&#x2F;pluto_highest_resolution_new_horizons_images_yet.html" rel="nofollow">http:&#x2F;&#x2F;www.slate.com&#x2F;blogs&#x2F;bad_astronomy&#x2F;2015&#x2F;12&#x2F;05&#x2F;pluto_hi...</a>