This is the largest and most upstream of the spring vents. It discharges horizontally into a large basin just east of the glass-bottom boat loading area. The vent is a horizontal, oval-shaped opening 2 to 6 feet high and about 65 feet wide at the base of a limestone wall. The bottom of the vent’s mouth is composed largely of breakdown boulders and rubble. The depth of water measured in front of the vent opening is about 34 feet. Water from at least two distinct sources (Mammoth East and Mammoth West) discharge from the cave opening. These sources differ in temperature and chemistry. The spring basin measures about 300 feet north to south and 200 feet east to west.