At least twice, Cistern was seen to erupt at the start of a major Steamboat eruption. These eruptions of Cistern reached as much as 20 feet. Click for more information. The color is due to the refraction of blue by the water combined with yellow from sulfur deposits lining the pools walls. Emerald is usually many degrees below boiling.
Bubbles seen rising to the surface are due to steam, carbon dioxide and other gasses.
This is not always true though. At times, especially at times of a disturbance , Emerald Spring can become turbid and begin to boil and sometimes even erupt to 3 feet or so. In 1931, Emerald saw spectacular activity. On that day, it exploded. The eruption emanating from a 2 inch vent in the large end of the opening.