Clear Air at Yellowstone Lake
Yellowstone Lake is a very large body of water located near the middle of the park. Due to the high elevation of the park, it is frozen much of the year, and filled with very cold water even in the summer -- cold enough that people are warned not to fall out of their boats! The lake is ringed with geothermal features, some of which are even under the water. It has been in the news lately because of evidence of geothermal activity in the lake bottom, with some scientists concerned about what this might mean in terms of the potential for earthquakes or eruptions in the greater park area.
I didn't take a lot of pictures of the lake, because, as one Yellowstone guide put it: "What can I say? It's a big lake." :) But I did like this view of one cove with these big rocks in the foreground.