Castle Geyser at Rest
Named for its terraced shape, Castle Geyser is one of the more beautiful formations at Yellowstone's Upper Geyser Basin. It's quite large, with a sinter cone over 15 feet high; the whole formation is about 50 feet wide (see the fence on the left side for some idea of scale.)
Castle is a fairly predictable geyser, but unfortunately we weren't around when it was erupting. I settled for this image of it in one of its more peaceful moods, its belching steam somehow mimicking the puffy clouds floating overhead.